cmp_null: make lint adhere to lint message convention

Author: matthiaskrgr

clippy_lints/src/ptr.rs

@@ -145,7 +145,7 @@ impl<'tcx> LateLintPass<'tcx> for Ptr {
                     cx,
                     CMP_NULL,
                     expr.span,
-                    "Comparing with null is better expressed by the `.is_null()` method",
+                    "comparing with null is better expressed by the `.is_null()` method",
                 );
             }
         }

tests/ui/cmp_null.stderr

@@ -1,12 +1,12 @@
-error: Comparing with null is better expressed by the `.is_null()` method
+error: comparing with null is better expressed by the `.is_null()` method
   --> $DIR/cmp_null.rs:9:8
    |
 LL |     if p == ptr::null() {
    |        ^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::cmp-null` implied by `-D warnings`
 
-error: Comparing with null is better expressed by the `.is_null()` method
+error: comparing with null is better expressed by the `.is_null()` method
   --> $DIR/cmp_null.rs:14:8
    |
 LL |     if m == ptr::null_mut() {