Fix up Contributor Covenant homepage link

There was a reference to a Markdown variable that has been deleted from this version, used the original Contributor Covenant as a reference.

Author: applecuckoo

CODE_OF_CONDUCT.md

@@ -123,7 +123,7 @@ accordingly.
 
 ## Attribution
 
-This Code of Conduct is adapted from the [Contributor Covenant][homepage],
+This Code of Conduct is adapted from the [Contributor Covenant](https://www.contributor-covenant.org),
 version 1.4, available at
 <https://www.contributor-covenant.org/version/1/4/code-of-conduct.html>,
 and the [Rust Code of Conduct](https://www.rust-lang.org/en-US/conduct.html).