Fix outdated comment about thread safety

While reading through this header file, I ran across this outdated
warning that said that use of function-level static initializers was not
thread-safe and hence not advisable for use with ThreadLocalStorage.
This comment was written before C++11 and function-level static
initializers were ensured to be constructed thread safely.

Change-Id: I8407edc5c32dc30e307a7192c42b411f22e75f7e
Reviewed-on: https://chromium-review.googlesource.com/c/1309300Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604392}

base/threading/thread_local.h

@@ -23,8 +23,7 @@
 //
 // Thread Safety: An instance of ThreadLocalStorage is completely thread safe
 // once it has been created. If you want to dynamically create an instance, you
-// must of course properly deal with safety and race conditions. This means a
-// function-level static initializer is generally inappropiate.
+// must of course properly deal with safety and race conditions.
 //
 // In Android, the system TLS is limited.
 //