docs(radio): fix typo in radio docs (#5842)

Fixes #5841.

Author: crisbeto

src/lib/radio/radio.md

@@ -1,18 +1,18 @@
 `<md-radio>` provides the same functionality as a native `<input type="radio">` enhanced with
-Material Design styling and animations.  
- 
+Material Design styling and animations.
+
 <!-- example(radio-overview) -->
- 
+
 All radio-buttons with the same `name` comprise a set from which only one may be selected at a time.
-  
+
 ### Radio-button label
-The radio-button label is provided as the content to the `<md-checkbox>` element. The label can be 
-positioned before or after the radio-button by setting the `labelPosition` property to `'before'` 
+The radio-button label is provided as the content to the `<md-radio-button>` element. The label can
+be positioned before or after the radio-button by setting the `labelPosition` property to `'before'`
 or `'after'`.
 
-If you don't want the label to appear next to the radio-button, you can use 
-[`aria-label`](https://www.w3.org/TR/wai-aria/states_and_properties#aria-label) or 
-[`aria-labelledby`](https://www.w3.org/TR/wai-aria/states_and_properties#aria-labelledby) to 
+If you don't want the label to appear next to the radio-button, you can use
+[`aria-label`](https://www.w3.org/TR/wai-aria/states_and_properties#aria-label) or
+[`aria-labelledby`](https://www.w3.org/TR/wai-aria/states_and_properties#aria-labelledby) to
 specify an appropriate label.
 
 
@@ -25,5 +25,5 @@ Individual radio-buttons inside of a radio-group will inherit the `name` of the
 
 
 ### Use with `@angular/forms`
-`<md-radio-group>` is compatible with `@angular/forms` and supports both `FormsModule` 
+`<md-radio-group>` is compatible with `@angular/forms` and supports both `FormsModule`
 and `ReactiveFormsModule`.