[Test] Update fix-it range for test case

Author: theblixguy

test/Sema/immutability.swift

@@ -159,7 +159,7 @@ struct SomeStruct {
       return 42
     }
     nonmutating
-    set {      // expected-note {{mark accessor 'mutating' to make 'self' mutable}} {{5-5=mutating }}
+    set {      // expected-note {{mark accessor 'mutating' to make 'self' mutable}} {{5-16=mutating}}
       iv = newValue // expected-error {{cannot assign to property: 'self' is immutable}}
     }
   }