Markdown syntax fix in CONTRIBUTING.md (prometheus#5671)

Signed-off-by: Jens Erat <[email protected]>

CONTRIBUTING.md

@@ -40,7 +40,7 @@ go build ./cmd/prometheus/
 make test         # Make sure all the tests pass before you commit and push :)
 ```
 
-We use `golangci-lint`[https://github.com/golangci/golangci-lint] for linting the code. If it reports an issue and you think that the warning needs to be disregarded or is a false-positive, you can add a special comment `//nolint:linter1[,linter2,...]` before the offending line. Use this sparingly though, fixing the code to comply with the linter's recommendation is in general the preferred course of action.
+We use [`golangci-lint`](https://github.com/golangci/golangci-lint) for linting the code. If it reports an issue and you think that the warning needs to be disregarded or is a false-positive, you can add a special comment `//nolint:linter1[,linter2,...]` before the offending line. Use this sparingly though, fixing the code to comply with the linter's recommendation is in general the preferred course of action.
 
 All our issues are regularly tagged so that you can also filter down the issues involving the components you want to work on. For our labeling policy refer [the wiki page](https://github.com/prometheus/prometheus/wiki/Label-Names-and-Descriptions).