Fix typo in extension_function.h

Fix typo occurence with occurrence in 1 file (first change to Chromium!)

Change-Id: I6edd6784ffd54a1f4bf0bc056692cfd89ad65ca7
Reviewed-on: https://chromium-review.googlesource.com/c/1257667Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595961}

extensions/browser/extension_function.h

@@ -347,7 +347,7 @@ class ExtensionFunction
   // Error. chrome.runtime.lastError.message will be set to |error|.
   ResponseValue Error(const std::string& error);
   // Error with formatting. Args are processed using
-  // ErrorUtils::FormatErrorMessage, that is, each occurence of * is replaced
+  // ErrorUtils::FormatErrorMessage, that is, each occurrence of * is replaced
   // by the corresponding |s*|:
   // Error("Error in *: *", "foo", "bar") <--> Error("Error in foo: bar").
   ResponseValue Error(const std::string& format, const std::string& s1);