Update README.rst

Author: rocky

README.rst

@@ -1,7 +1,7 @@
 Test Mathics3 Module
 
 This is a Mathics3 Python Module showing the classic "Hello, World!"
-The purpose is to demonstrating how to write a Mathics3 Python Module which extends Mathics3 by adding a function but written in Python.
+The purpose is to demonstrate how to write a Mathics3 Python Module which extends Mathics3 by adding a function, but written in Python.
 
 You can also use this as a template to clone if you want to create your own Mathics3 Module.