Update docs/how-to/heterogeneous_matches.rst

Co-authored-by: Vince Knight <[email protected]>

Author: zewaywong

docs/how-to/heterogeneous_matches.rst

@@ -1,7 +1,7 @@
 .. _heterogeneous-matches:
 
 Use custom matches
-===================
+==================
 
 Axelrod Matches are homogeneous by nature but can be extended to utilize additional attributes of heterogeneous players. 
 This tutorial indicates how the Axelrod :code:`Match` class can be manipulated in order to play heterogeneous tournaments and Moran processes using country mass as a score modifier.