Bump Safari 10.1 to Safari 11 in grade C browser list

The summary includes Safari 10.1 for macOS as an example of a Grade C browser.

However in the Grade C section of the document it's mentioned that Safari 10.1 will not run our JavaScript as (although it technically supports `type="module"`) it fails the prerequisite support tests present in Frontend. That section lists Safari 11 as being Grade C instead.

Updating the summary, presuming that Safari 10.1 is actually Grade X due to not receiving any JavaScript enhancements.

docs/contributing/browser-support.md

@@ -15,7 +15,7 @@ You can view a summary of [the grades and the support we provide](https://fronte
 
 - **grade A** - Most recent stable versions of Chrome, Firefox, Edge, Samsung Internet and Safari
 - **grade B** - All stable versions of Chrome, Firefox and Edge released in the last 6 months and all the major stable releases of Safari which are not supported in Grade A
-- **grade C** - [All browsers that support `<script type="module">`](https://caniuse.com/es6-module) (Chrome 61+, Edge 16-18, Edge 79+, Safari 10.1+ (mac), Firefox 60+, Opera 48+, Safari 10.3+ (iOS), Samsung Internet 8.2+)
+- **grade C** - [All browsers that support `<script type="module">`](https://caniuse.com/es6-module) (Chrome 61+, Edge 16-18, Edge 79+, Safari 11+ (macOS), Firefox 60+, Opera 48+, Safari 10.3+ (iOS), Samsung Internet 8.2+)
 - **grade X** - All other browsers (including IE11 and older)
 
 > **Note: Only browsers in grades A, B and C will run our JavaScript enhancements. We will not support our JavaScript enhancements for older browsers in grade X.**