[Buildbot] #3201: Update example for using codebases with the force scheduler

[Buildbot trac]

#3201: Update example for using codebases with the force scheduler
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Reporter:  dustin       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  minor        |  Milestone:  0.9.+
 Version:  master       |   Keywords:  docs
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 In particular, we've now deprecated the `branch` argument, preferring to
 use codebases -- but the example in the documentation still lists a
 `branch` argument.

 This will need some research and experimentation to get the new
 configuration right to give the same behavior as the old.

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Ticket URL: <http://trac.buildbot.net/ticket/3201>
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