Merge pull request twbs#15862 from dante2008/contri-doc-typo

Fix a typo in the contributing doc.

CONTRIBUTING.md

@@ -33,7 +33,7 @@ restrictions:
 
 Our bug tracker utilizes several labels to help organize and identify issues. Here's what they represent and how we use them:
 
-- `browser bug` - Issues that are reported to us, but actually are the result of a browser-specific bug. These are diagnosed with reduced test cases and result in a issue opened on that browser's own bug tracker.
+- `browser bug` - Issues that are reported to us, but actually are the result of a browser-specific bug. These are diagnosed with reduced test cases and result in an issue opened on that browser's own bug tracker.
 - `confirmed` - Issues that have been confirmed with a reduced test case and identify a bug in Bootstrap.
 - `css` - Issues stemming from our compiled CSS or source Less files.
 - `customizer` - Issues with our web-based Customizer.