Fix sphinx warning about title underline begin too short

[tareqsiraj]

Fix already committed in rL191211

[jtsoftware]

Thanks, Tareq. I only found out and fixed it because of your post, as it
doesn't warn on my system, so thanks!

-John

docs/ModularizeUsage.rst

 
 ``<front-end-options>`` is a place-holder for regular Clang
 front-end arguments, which must follow the <include-files-list>.
 Note that by default, the underlying Clang front end assumes .h files
 contain C source, so you might need to specify the ``-x c++`` Clang option
 to tell Clang that the header contains C++ definitions.
 
 Modularize Command Line Options
-============================
+===============================
 
 .. option:: -prefix <header-path>
 
   Prepend the given path to non-absolute file paths in the header list file.
   By default, headers are assumed to be relative to the header list file
   directory.  Use -prefix to specify a different directory.