git-gui: add missing close bracket

In d9c6469 (git-gui: update status bar to track operations, 2019-12-01),
the status bar was refactored to allow multiple overlapping operations.

Since the refactor changed the status bar interface, all callsites had
to be refactored to use the new interface. During that refactoring, this
closing bracket was missed. This leads to an error message popping up
when doing 'Branch->Reset...'.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

lib/merge.tcl

@@ -244,7 +244,7 @@ Continue with resetting the current changes?"]
 		set status_bar_operation [$::main_status \
 			start \
 			[mc "Aborting"] \
-			[mc "files reset"]
+			[mc "files reset"]]
 		fileevent $fd readable [namespace code [list \
 			_reset_wait $fd $status_bar_operation]]
 	} else {