Update README.rst

Co-Authored-By: robbuckley <20515024+robbuckley@users.noreply.github.com>

README.rst

@@ -64,7 +64,7 @@ shell scripts listed above are available for your build and test.
 OS X builds may be targetted either at macOS 10.6+
 (dual arch 64 / 32 bit) or macOS 10.9+ (64b only). These depend on the
 corresponding build of python from https://www.python.org/downloads/mac-osx/.
-At the time of writing, 10.9+ / 64 bit build are supported for python
+At the time of writing, 10.9+ / 64 bit builds are supported for python
 versions 3.6.5 / 2.7.15 and above. If you want to build for an older version
 of python, you'll have to target 10.6+ / dual arch.