mem_replace: fix grammar.

clippy_lints/src/mem_replace.rs

@@ -54,7 +54,7 @@ fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
             then {
                 // Since this is a late pass (already type-checked),
                 // and we already know that the second argument is an
-                // `Option`, we do not need to check if the first
+                // `Option`, we do not need to check the first
                 // argument's type. All that's left is to get
                 // replacee's path.
                 let replaced_path = match func_args[0].node {