doc: Fix a few minor ReST issues

Sphinx was complaining about too-short title underlines.

Author: bgamari

doc/common-errors.rst

@@ -7,7 +7,7 @@ Common Errors
 This is probably caused by the ``-- | xxx`` comment not following a declaration. I.e. use ``-- xxx`` instead. See :ref:`top-level-declaration`.
 
 ``parse error on input ‘-- $ xxx’``
-----------------------------------
+-----------------------------------
 
 You've probably commented out code like::
 

doc/markup.rst

@@ -950,8 +950,8 @@ apostrophes themselves: to hyperlink ``foo'`` one would simply type
 
     -- | A prefix operator @'(++)'@ and an infix identifier @'`elem`'@.
 
-Emphasis, Bold and Monospaced styled Text
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Emphasis, Bold and Monospaced Styled Text
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Text can be emphasized, made bold (strong) or monospaced (typewriter font) 
 by surrounding it with slashes, double-underscores or at-symbols: ::