docs: add indefinite article to bug fix sentence (#1878)

Author: Bamblehorse

.github/contributing.md

@@ -23,7 +23,7 @@ Hi! I'm really excited that you are interested in contributing to Vue.js. Before
   - Add accompanying test case.
   - Provide a convincing reason to add this feature. Ideally, you should open a suggestion issue first and have it approved before working on it.
 
-- If fixing bug:
+- If fixing a bug:
 
   - If you are resolving a special issue, add `(fix #xxxx[,#xxxx])` (#xxxx is the issue id) in your PR title for a better release log, e.g. `update entities encoding/decoding (fix #3899)`.
   - Provide a detailed description of the bug in the PR. Live demo preferred.