Update "C-Bug" label and url in CONTRIBUTING.md (bevyengine#4880)

'bug' is not a valid label. Changed it to "C-Bug".

Co-authored-by: siph <siph@github>

CONTRIBUTING.md

@@ -198,7 +198,7 @@ Check out our [plugin guidelines](https://github.com/bevyengine/bevy/blob/main/d
 
 ### Fixing bugs
 
-Bugs in Bevy (or the associated website / book) are filed on the issue tracker using the [`bug`](https://github.com/bevyengine/bevy/issues?q=is%3Aissue+is%3Aopen+label%3Abug) label.
+Bugs in Bevy (or the associated website / book) are filed on the issue tracker using the [`C-Bug`](https://github.com/bevyengine/bevy/issues?q=is%3Aissue+is%3Aopen+label%3AC-Bug) label.
 
 If you're looking for an easy place to start, take a look at the [`E-Good-First-Issue`](https://github.com/bevyengine/bevy/issues?q=is%3Aopen+is%3Aissue+label%3AE-Good-First-Issue) label, and feel free to ask questions on that issue's thread in question or on Discord.
 You don't need anyone's permission to try fixing a bug or adding a simple feature, but stating that you'd like to tackle an issue can be helpful to avoid duplicated work.