Fix typo in error message (#3848)

rts/RtsAPI.c

@@ -541,7 +541,7 @@ rts_lock (void)
     if (running_finalizers) {
         errorBelch("error: a C finalizer called back into Haskell.\n"
                    "   This was previously allowed, but is disallowed in GHC 6.10.2 and later.\n"
-                   "   To create finalizers that may call back into Haskll, use\n"
+                   "   To create finalizers that may call back into Haskell, use\n"
                    "   Foreign.Concurrent.newForeignPtr instead of Foreign.newForeignPtr.");
         stg_exit(EXIT_FAILURE);
     }