[base] Fix typo in task_environment.cc comment

TBR=fdoray@chromium.org

Change-Id: Ic54f6e6e3935ec0e562da95eb76fd2d49c1b7b64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560808Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831115}

base/test/task_environment.cc

@@ -309,7 +309,7 @@ class TaskEnvironment::MockTimeDomain : public sequence_manager::TimeDomain,
         // Thread B : Complete enqueue of task U.
         // Thread A : FastForwardToNextTaskOrCap() => must stay at 300ms and run
         //            U, not go back to 120ms.
-        // Hence we need std::max() to protect again this because construction
+        // Hence we need std::max() to protect against this because construction
         // and enqueuing isn't atomic in time (LazyNow support in
         // base/task/thread_pool could help).
         now_ticks_ = std::max(now_ticks_, *next_task_time);