doc: reword "config file" to "configuration file"

While we do use the abbreviation "config" elsewhere in the docs, the
phrase "configuration file" is always written out in full, except in
this location in phases.txt.

mercurial/help/phases.txt

@@ -52,7 +52,7 b' repository to disable publishing in its '
   [phases]
   publish = False
 
-See :hg:`help config` for more information on config files.
+See :hg:`help config` for more information on configuration files.
 
 .. note::
   Servers running older versions of Mercurial are treated as