Fix a typo in a comment for Closure.

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BUG=

Review URL: https://codereview.chromium.org/362343007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281181 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/callback.h b/base/callback.h
index 5a4e879..364f5061 100644
--- a/base/callback.h
+++ b/base/callback.h
@@ -761,7 +761,7 @@
 };
 
 
-// Syntactic sugar to make Callbacks<void(void)> easier to declare since it
+// Syntactic sugar to make Callback<void(void)> easier to declare since it
 // will be used in a lot of APIs with delayed execution.
 typedef Callback<void(void)> Closure;