help: clarify that the config hook priority prefix includes a period

mercurial/help/config.txt

@@ -734,7 +734,7 various actions such as starting or fini
 hooks can be run for the same action by appending a suffix to the
 action. Overriding a site-wide hook can be done by changing its
 value or setting it to an empty string.  Hooks can be prioritized
-by adding a prefix of ``priority`` to the hook name on a new line
+by adding a prefix of ``priority.`` to the hook name on a new line
 and setting the priority. The default priority is 0.
 
 Example ``.hg/hgrc``::