doc(input): fix small typo in code example

Author: hamstu

src/ng/directive/input.js

@@ -739,7 +739,7 @@ function checkboxInputType(scope, element, attr, ctrl) {
          <tt>myForm.userName.$valid = {{myForm.userName.$valid}}</tt><br>
          <tt>myForm.userName.$error = {{myForm.userName.$error}}</tt><br>
          <tt>myForm.lastName.$valid = {{myForm.lastName.$valid}}</tt><br>
-         <tt>myForm.userName.$error = {{myForm.lastName.$error}}</tt><br>
+         <tt>myForm.lastName.$error = {{myForm.lastName.$error}}</tt><br>
          <tt>myForm.$valid = {{myForm.$valid}}</tt><br>
          <tt>myForm.$error.required = {{!!myForm.$error.required}}</tt><br>
          <tt>myForm.$error.minlength = {{!!myForm.$error.minlength}}</tt><br>