chore: Update README to correctly show supported python versions (aws#335)

We had a copy/paste error and were pointing at the CLI

Author: farleyb-amazon

README.rst

@@ -6,7 +6,7 @@ aws-encryption-sdk
    :target: https://pypi.python.org/pypi/aws-encryption-sdk
    :alt: Latest Version
 
-.. image:: https://img.shields.io/pypi/pyversions/aws-encryption-sdk-cli.svg
+.. image:: https://img.shields.io/pypi/pyversions/aws-encryption-sdk.svg
    :target: https://pypi.python.org/pypi/aws-encryption-sdk
    :alt: Supported Python Versions