Auto merge of #4501 - derivmug:update-lint-doc, r=phansch
Auto merge of #4501 - derivmug:update-lint-doc, r=phansch

Remove ui/ from TESTNAME for foo lint example

changelog: Remove ui/ from TESTNAME in doc/adding-lints.md

This PR remove the `ui/` from the TESTNAME of the example foo lint in the 'Adding lints' documentation.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust