Remove superfluous "either"

chrome/test/data/extensions/api_test/README.txt

 This directory contains extensions which are unit tests for the extension API.
 These tests are written using the extension API test framework, which allows
 us to do end-to-end testing of extension API in a browser_test.  The general way
 these tests work is to run code in an extension when they're loaded and to post
 a pass or fail notification back up to the C++ unit test which then reports the
 success or failure.  In the common case, the extension runs many subtests and
 then reports up a single pass or fail.  This case is made easy by the test
 framework.
 
 To write a new test:
 
 (1) Add a new browser_test which is a subclass of ExtensionApiTest.  This test
 should call RunExtensionTest("extension_name") to kick off the test.  See
 bookmark_extension_apitest.cc for an example.
 
 (2) Create an extension of in this directory of the same name as the extension
 that your test referred to ("extension_name" above).  This test should load
-either a background page which immediately starts its test.
+a background page which immediately starts its test.
 
 (3) In your extension page, call chrome.test.runTests with an array of
 functions which represent your subtests.  Each of these functions will most
 likely call one or more async extension APIs.  Wrap the callback for each of
 these API calls with chrome.test.callbackPass or chrome.test.callbackFail
 depending on whether or not you're expecting the callback to generate an error
 or not.  That's it.  The test framework notices when each of these callbacks
 is registered and keeps a count of what's expected.  When the right number of
 callbacks has fired, that test function will be marked as passed or failed and
 the next one will be called.  Some other useful helper functions you'll use are
@@ skipping 51 matching lines @@
 
 [----------] Global test environment tear-down
 [==========] 1 test from 1 test case ran. (2482 ms total)
 [  PASSED  ] 1 test.
 1 test run
 0 test failed
 
 Note the RUN/SUCCESS messages in () - these are the subtests that are run in
 the extension itself.  Anything printed with chrome.test.log() will also display
 in stdout of the browser test (and hence in the buildbot output for that test).