yat2m: Use \fP to return previous font.

* doc/yat2m.c (proc_texi_cmd): Use \fP instead of \fR.

This can fix the output of following input (in gpg.texi):

@item --list-filter @{select=@var{expr}@}

The last right brace is in bold font.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>