Update installation.rst with Python3 print() syntax (datastax#1204)

Author: bschoening

docs/installation.rst

@@ -24,7 +24,7 @@ Verifying your Installation
 ---------------------------
 To check if the installation was successful, you can run::
 
-    python -c 'import cassandra; print cassandra.__version__'
+    python -c 'import cassandra; print(cassandra.__version__)'
 
 It should print something like "3.29.1".