[Buildbot] #3432: normalize index entries for "BuildSlaves" in docs

[Buildbot trac]

#3432: normalize index entries for "BuildSlaves" in docs
----------------------+-----------------------
Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  0.8.12     |   Keywords:
----------------------+-----------------------
 Currently `BuildSlaves` category in index is represented using three
 names: `Build Slaves`, `Buildslaves`, `BuildSlaves` (see
 http://docs.buildbot.net/latest/genindex.html):

 {{{
 Build Slaves

     limiting concurrency

 ...

 BuildSlaves

     AWS EC2
     DB Connector Component
     latent, [1]
     libvirt

 Buildslaves

     Docker

 }}}

 I will unite them under `Worker` name (in #2340), if there is no
 objections.

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Ticket URL: <http://trac.buildbot.net/ticket/3432>
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