update date-check docs

Author: tshepang

Diff for: src/contributing.md

@@ -440,15 +440,8 @@ Just a few things to keep in mind:
   - The date the comment was added, e.g. instead of writing _"Currently, ..."_
     or _"As of now, ..."_, consider writing
     _"As of January 2021, ..."_.
-    Try to format the date as `<MONTH> <YEAR>` to ease search.
-
-  - Additionally, include a machine-readable comment of the form `<!-- date:
-    2022-04 -->` (if the current month is April 2022). We have an automated
-    tool that uses these (in `ci/date-check`).
-
-    So, for the month of April 2022, the comment would look like: `As of <!--
-    date: 2022-04 --> April 2022`. Make sure to put the comment *between* `as of`
-    and `April 2022`; see [PR #1066][rdg#1066] for the rationale.
+    We have CI action (in `"~/.github/workflows/date-check.yml"`)
+    that generates an issue if any of these are over 6 months old.
 
   - A link to a relevant WG, tracking issue, `rustc` rustdoc page, or similar, that may provide
     further explanation for the change process or a way to verify that the information is not
@@ -460,7 +453,6 @@ Just a few things to keep in mind:
 
 [rdg]: https://rustc-dev-guide.rust-lang.org/
 [rdgrepo]: https://github.com/rust-lang/rustc-dev-guide
-[rdg#1066]: https://github.com/rust-lang/rustc-dev-guide/pull/1066
 
 ## Issue Triage