Fix short underline in docs

Author: henryiii

docs/advanced/pycpp/utilities.rst

@@ -24,7 +24,7 @@ expected in Python:
 .. _eval:
 
 Capturing ``std::cout`` and ``std::cerr`` usage
-===========================================
+===============================================
 
 Often, a library may use ``std::cout`` and ``std::cerr`` to print, but this does
 not play well with Python's standard ``sys.stdout`` and ``sys.stderr`` redirection.