Fix trivial typo in Readme (GH-2185)

Replace platform with platforms.

Author: csabella

Doc/using/index.rst

@@ -6,7 +6,7 @@
 
 
 This part of the documentation is devoted to general information on the setup
-of the Python environment on different platform, the invocation of the
+of the Python environment on different platforms, the invocation of the
 interpreter and things that make working with Python easier.