docs(form): Fix typos of indent

Closes #5159

modules/angular2/src/common/forms/model.ts

@@ -244,7 +244,7 @@ export abstract class AbstractControl {
  * `Control` is one of the three fundamental building blocks used to define forms in Angular, along
  * with {@link ControlGroup} and {@link ControlArray}.
  *
- *##Usage
+ * ##Usage
  *
  * By default, a `Control` is created for every `<input>` or other form component.
  * With {@link NgFormControl} or {@link NgFormModel} an existing {@link Control} can be
@@ -420,7 +420,7 @@ export class ControlGroup extends AbstractControl {
  * along with {@link Control} and {@link ControlGroup}. {@link ControlGroup} can also contain
  * other controls, but is of fixed length.
  *
- *##Adding or removing controls
+ * ##Adding or removing controls
  *
  * To change the controls in the array, use the `push`, `insert`, or `removeAt` methods
  * in `ControlArray` itself. These methods ensure the controls are properly tracked in the
@@ -488,4 +488,4 @@ export class ControlArray extends AbstractControl {
   _setParentForControls(): void {
     this.controls.forEach((control) => { control.setParent(this); });
   }
-}
+}