Fixed a typo.

support/mnt-excl

@@ -20,7 +20,7 @@
 # ADDENDUM:  The addition of the --filter option (which has support for
 # absolute-anchored excludes) has made this script less useful than it
 # was.  Beginning with 2.6.4, you can achieve the effect of this script
-# though this command:
+# through this command:
 #
 # awk '{print $2}' /proc/mounts | rsync -f 'merge,/- -' host:/dir /dest/