[Extensions] Getting started patch to fix a typo

There are other identical typos.
However, those can remain for other on-boarding.

Bug: None

Change-Id: Ie04e83346de7906af5c4ab5ce2b95ac56ce99ade
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276499Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Solomon Kinard <solomonkinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784171}

extensions/test/data/icon_visibility/README

@@ -2,7 +2,7 @@ This file explains the purpose of this directory and how to set everything up
 to run the related unit test to verify the icon visibility algorithm is
 working reasonably well.
 
-The unit test is in src/extesions/common/image_util_unittest.cc and is
+The unit test is in src/extensions/common/image_util_unittest.cc and is
 disabled, so it doesn't run under normal testing:
 
 TEST(ImageUtilTest, DISABLED_AnalyzeAllDownloadedIcons)