fix typos

CHANGES.rst

@@ -20,10 +20,10 @@ Unreleased
     :issue:`2638`
 -   Cache the help option generated by the ``help_option_names`` setting to
     respect its eagerness. :pr:`2811`
--   Replace uses of `os.system` with `subprocess.Popen`. :issue:`1476`
+-   Replace uses of ``os.system`` with ``subprocess.Popen``. :issue:`1476`
 -   Exceptions generated during a command will use the context's ``color``
     setting when being displayed. :issue:`2193`
--   Error message when definig option with invalid name is more descriptive.
+-   Error message when defining option with invalid name is more descriptive.
     :issue:`2452`
 -   Refactor code generating default ``--help`` option to deduplicate code.
     :pr:`2563`