Fixing typo in the docstring for bisect_perf_regression.py

tools/auto_bisect/bisect_perf_regression.py

 #!/usr/bin/env python
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Chromium auto-bisect tool
 
 This script bisects a range of commits using binary search. It starts by getting
 reference values for the specified "good" and "bad" commits. Then, for revisions
 in between, it will get builds, run tests and classify intermediate revisions as
 "good" or "bad" until an adjacent "good" and "bad" revision is found; this is
 the culprit.
 
-If the culprit is a roll if a depedency repository (e.g. v8), it will then
+If the culprit is a roll of a depedency repository (e.g. v8), it will then
 expand the revision range and continue the bisect until a culprit revision in
 the dependency repository is found.
 
 Example usage using git commit hashes, bisecting a performance test based on
 the mean value of a particular metric:
 
 ./tools/auto_bisect/bisect_perf_regression.py
   --command "out/Release/performance_ui_tests \
       --gtest_filter=ShutdownTest.SimpleUserQuit"\
   --metric shutdown/simple-user-quit
@@ skipping 2795 matching lines @@
       # bugs. If you change this, please update the perf dashboard as well.
       bisect_utils.OutputAnnotationStepStart('Results')
     print 'Runtime Error: %s' % e
     if opts.output_buildbot_annotations:
       bisect_utils.OutputAnnotationStepClosed()
   return 1
 
 
 if __name__ == '__main__':
   sys.exit(main())