Fix missing parenthesis in suboptimal floating point help

tests/ui/floating_point_mul_add.fixed

@@ -33,6 +33,9 @@ fn main() {
 
     let _ = a.mul_add(a, b).sqrt();
 
+    let u = 1usize;
+    let _ = b.mul_add(-(u as f64), a);
+
     // Cases where the lint shouldn't be applied
     let _ = (a * a + b * b).sqrt();
 }

tests/ui/floating_point_mul_add.rs

@@ -33,6 +33,9 @@ fn main() {
 
     let _ = (a * a + b).sqrt();
 
+    let u = 1usize;
+    let _ = a - (b * u as f64);
+
     // Cases where the lint shouldn't be applied
     let _ = (a * a + b * b).sqrt();
 }

tests/ui/floating_point_mul_add.stderr

@@ -73,5 +73,11 @@ error: multiply and add expressions can be calculated more efficiently and accur
 LL |     let _ = (a * a + b).sqrt();
    |             ^^^^^^^^^^^ help: consider using: `a.mul_add(a, b)`
 
-error: aborting due to 12 previous errors
+error: multiply and add expressions can be calculated more efficiently and accurately
+  --> $DIR/floating_point_mul_add.rs:37:13
+   |
+LL |     let _ = a - (b * u as f64);
+   |             ^^^^^^^^^^^^^^^^^^ help: consider using: `b.mul_add(-(u as f64), a)`
+
+error: aborting due to 13 previous errors