Typo Fix `necssary`

created a pull request here https://github.com/ghc/ghc/pull/72

diff --git a/compiler/hsSyn/HsExpr.hs b/compiler/hsSyn/HsExpr.hs
index 1bde776..f75cc2c 100644
--- a/compiler/hsSyn/HsExpr.hs
+++ b/compiler/hsSyn/HsExpr.hs
@@ -740,7 +740,7 @@ HsPar (and ParPat in patterns, HsParTy in types) is used as follows     

   * ParPat and HsParTy are pretty printed as '( .. )' regardless of whether or
-    not they are strictly necssary. This should be addressed when #13238 is
+    not they are strictly necessary. This should be addressed when #13238 is

     completed, to be treated the same as HsPar.