docs($http): fix adding default header to get request example

Initially, `$httpProvider.defaults.headers.get` is `undefined`, so
`$httpProvider.defaults.headers.get['My-Header']='value'` will throw an
error.

Closes angular#4101

Author: MrRaindrop

src/ng/http.js

@@ -251,7 +251,7 @@ function $HttpProvider() {
      * To add or overwrite these defaults, simply add or remove a property from these configuration
      * objects. To add headers for an HTTP method other than POST or PUT, simply add a new object
      * with the lowercased HTTP method name as the key, e.g.
-     * `$httpProvider.defaults.headers.get['My-Header']='value'`.
+     * `$httpProvider.defaults.headers.get = { 'My-Header' : 'value' }.
      *
      * Additionally, the defaults can be set at runtime via the `$http.defaults` object in the same
      * fashion.