docs(ngModel): improve formatting of $modelValue

Closes angular#11483

Author: LeonardoBraga

src/ng/directive/ngModel.js

@@ -504,7 +504,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    * If the validity changes to invalid, the model will be set to `undefined`,
    * unless {@link ngModelOptions `ngModelOptions.allowInvalid`} is `true`.
    * If the validity changes to valid, it will set the model to the last available valid
-   * modelValue, i.e. either the last parsed value or the last value set from the scope.
+   * `$modelValue`, i.e. either the last parsed value or the last value set from the scope.
    */
   this.$validate = function() {
     // ignore $validate before model is initialized