Fix ungrammatical error message

Test Plan: If it builds, ship it

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3772

compiler/rename/RnSource.hs

@@ -964,7 +964,7 @@ rnSrcDerivDecl (DerivDecl ty deriv_strat overlap)
        ; unless standalone_deriv_ok (addErr standaloneDerivErr)
        ; failIfTc (isJust deriv_strat && not deriv_strats_ok) $
            illegalDerivStrategyErr $ fmap unLoc deriv_strat
-       ; (ty', fvs) <- rnLHsInstType (text "In a deriving declaration") ty
+       ; (ty', fvs) <- rnLHsInstType (text "a deriving declaration") ty
        ; return (DerivDecl ty' deriv_strat overlap, fvs) }
 
 standaloneDerivErr :: SDoc

testsuite/tests/partial-sigs/should_fail/WildcardInStandaloneDeriving.stderr

@@ -3,4 +3,4 @@ WildcardInStandaloneDeriving.hs:4:19: error: Malformed instance: _
 
 WildcardInStandaloneDeriving.hs:4:19: error:
     Wildcard ‘_’ not allowed
-      in In a deriving declaration
+      in a deriving declaration