CONTRIBUTING.md: Adjust link to "submitting patches" instructions.

The link added in commit a2a0c50c was pointing to old documentation
according to dbaio@.

Reported by:	dbaio

CONTRIBUTING.md

 FreeBSD does not currently accept pull requests. This is a read-only mirror from SVN.
 
-Please see the [Porter's Handbook](https://docs.freebsd.org/en/books/porters-handbook/) section on [Submitting patches](https://docs.freebsd.org/en/books/porters-handbook/porting-submitting.html).
+Please see the [Porter's Handbook](https://docs.freebsd.org/en/books/porters-handbook/) section on [Submitting patches](https://docs.freebsd.org/en/books/porters-handbook/quick-porting/#porting-submitting).