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.
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors