ref_in_deref: make lint adhere to lint message convention

clippy_lints/src/reference.rs

@@ -92,7 +92,7 @@ fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &Expr) {
                     cx,
                     REF_IN_DEREF,
                     object.span,
-                    "Creating a reference that is immediately dereferenced.",
+                    "creating a reference that is immediately dereferenced",
                     "try this",
                     snippet_with_applicability(cx, inner.span, "_", &mut applicability).to_string(),
                     applicability,

tests/ui/unnecessary_ref.stderr

@@ -1,4 +1,4 @@
-error: Creating a reference that is immediately dereferenced.
+error: creating a reference that is immediately dereferenced
   --> $DIR/unnecessary_ref.rs:13:17
    |
 LL |     let inner = (&outer).inner;