Fix package name in intro docs to reflect actual pypi name (with capitalization)

Author: kevin-reynolds-roblox

doc/source/intro.rst

@@ -32,7 +32,7 @@ installed, just run the following from the command-line:
 
 .. sourcecode:: none
 
-    # pip install gitpython
+    # pip install GitPython
 
 This command will download the latest version of GitPython from the
 `Python Package Index <http://pypi.python.org/pypi/GitPython>`_ and install it