[GHC] #15406: Fix a small typo in TcType.hs

[GHC]

#15406: Fix a small typo in TcType.hs
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           Reporter:  v0d1ch         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Documentation  |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 >>  not substTy, bucause the latter uses smart constructors that do
 s/bucause/because

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15406>
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