[iOS] Fix TabModelObserver method name in BVC

This CL fixes a one-character typo in the method name of one of the new
TabModelObserver methods in the BVC.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I1b1f72d4053452a3e11c6b283f99b8214def4d39
Reviewed-on: https://chromium-review.googlesource.com/847006
Reviewed-by: edchin <[email protected]>
Commit-Queue: edchin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#526517}
1 file changed