doc: correct example concerning "hg purge" alias in man page "hgrc.5"

The "hg purge" alias as currently described in "hgrc.5" issues a possibly
confusing error message like

   rm: missing operand
   Try 'rm --help' for more information.

if no files are to be purged at all.

This patch slightly modifies the example by adding a "-f" option to the
"rm" command.

mercurial/help/config.txt

@@ -277,7 +277,7 run arbitrary commands. As an example, :
 will let you do ``hg echo foo`` to have ``foo`` printed in your
 terminal. A better example might be::
 
-   purge = !$HG status --no-status --unknown -0 re: | xargs -0 rm
+   purge = !$HG status --no-status --unknown -0 re: | xargs -0 rm -f
 
 which will make ``hg purge`` delete all unknown files in the
 repository in the same manner as the purge extension.