[GHC] #13079: Fixed typo in docs

[GHC]

#13079: Fixed typo in docs
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           Reporter:  forki          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  lowest         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Documentation
  Unknown/Multiple                   |  bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13079>
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