Fix trivial typo in Readme (GH-2185)
Replace platform with platforms.
diff --git a/Doc/using/index.rst b/Doc/using/index.rst
index a5df713..4f0aa7d 100644
--- a/Doc/using/index.rst
+++ b/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.