update Exercise 8.1

change "istream" into "std::istream"

ch08/README.md

@@ -4,7 +4,7 @@
 >Write a function that takes and returns an istream&. The function should read the stream until it hits end-of-file. The function should print what it reads to the standard output. Reset the stream so that it is valid before returning the stream.
 
 ```cpp
-istream& func(istream &is)
+std::istream& func(std::istream &is)
 {
     std::string buf;
     while (is >> buf)