fsmonitor: update documentation to remove reference to invalid config settings

Remove the reference to setting core.fsmonitor to `true` (or `false`) as those
are not valid settings.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -484,8 +484,8 @@ the `core.fsmonitor` configuration variable (see
 linkgit:git-config[1]) than using the `--fsmonitor` option to
 `git update-index` in each repository, especially if you want to do so
 across all repositories you use, because you can set the configuration
-variable to `true` (or `false`) in your `$HOME/.gitconfig` just once
+variable in your `$HOME/.gitconfig` just once and have it affect all
-and have it affect all repositories you touch.
+repositories you touch.
 
 When the `core.fsmonitor` configuration variable is changed, the
 file system monitor is added to or removed from the index the next time