Rename subdir doc to docs where appropriate

This makes the subdir naming consistent across modules.

Author: bogdanr

gr-analog/CMakeLists.txt

@@ -90,7 +90,7 @@ CPACK_COMPONENT("analog_swig"
 ########################################################################
 add_subdirectory(include/gnuradio/analog)
 add_subdirectory(lib)
-add_subdirectory(doc)
+add_subdirectory(docs)
 if(ENABLE_PYTHON)
     add_subdirectory(swig)
     add_subdirectory(python/analog)