Fix link: hyphen should be underscore.

README.rst

@@ -55,7 +55,7 @@ For more help on using Mesa, check out the following resources:
 * `Email list`_
 * `PyPI`_
 
-.. _`Intro to Mesa Tutorial` : http://mesa.readthedocs.org/en/latest/tutorials/intro-tutorial.html
+.. _`Intro to Mesa Tutorial` : http://mesa.readthedocs.org/en/latest/tutorials/intro_tutorial.html
 .. _`Docs` : http://mesa.readthedocs.org/en/latest/
 .. _`Email list` : https://groups.google.com/d/forum/projectmesa
 .. _`PyPI` : https://pypi.python.org/pypi/Mesa/