Update readme.rst (esp8266#4131)

* Update readme.rst

fix broken link to file system flash layout

* linkage repair

my second shot at fixing the external page link for "flash layout". It appears that the on-page link to "Update Process - Memory View" was broken (not sure if I got this one right or not).

doc/ota_updates/readme.rst

@@ -57,15 +57,15 @@ Basic Requirements
 
 Flash chip size should be able to hold the old sketch (currently running) and the new sketch (OTA) at the same time.
 
-Keep in mind that the File system and EEPROM for example needs space too (one time) see `flash layout <../filesystem.rst#flash-layout>`__.
+Keep in mind that the File system and EEPROM for example needs space too (one time) see :ref:`Flash layout`.
 
 .. code:: cpp
 
     ESP.getFreeSketchSpace();
 
 can be used for checking the free space for the new sketch.
 
-For overview of memory layout, where new sketch is stored and how it is copied during OTA process, see `Update process - memory view <#update-process---memory-view>`__.
+For overview of memory layout, where new sketch is stored and how it is copied during OTA process, see `Update process - memory view <#update-process-memory-view>`__.
 
 The following chapters provide more details and specific methods of doing OTA.
 
@@ -224,9 +224,9 @@ Instead of the log as on the above screen you may see the following:
 
 If this is the case, then most likely ESP module has not been reset after initial upload using serial port.
 
-The most common causes of OTA failure are as follows: \* not enough physical memory on the chip (e.g. ESP01 with 512K flash memory is not enough for OTA), \* too much memory declared for SPIFFS so new sketch will not fit between existing sketch and SPIFFS – see `Update process - memory view <#update-process---memory-view>`__, \* too little memory declared in Arduino IDE for your selected board (i.e. less than physical size), \* not resetting the ESP module after initial upload using serial port.
+The most common causes of OTA failure are as follows: \* not enough physical memory on the chip (e.g. ESP01 with 512K flash memory is not enough for OTA), \* too much memory declared for SPIFFS so new sketch will not fit between existing sketch and SPIFFS – see `Update process - memory view <#update-process-memory-view>`__, \* too little memory declared in Arduino IDE for your selected board (i.e. less than physical size), \* not resetting the ESP module after initial upload using serial port.
 
-For more details regarding flash memory layout please check `File system <../filesystem.rst>`__. For overview where new sketch is stored, how it is copied and how memory is organized for the purpose of OTA see `Update process - memory view <#update-process---memory-view>`__.
+For more details regarding flash memory layout please check `File system <../filesystem.rst>`__. For overview where new sketch is stored, how it is copied and how memory is organized for the purpose of OTA see `Update process - memory view <#update-process-memory-view>`__.
 
 Web Browser
 -----------