Update contributors workspace link in CONTRIBUTING.rst. (#760)

* Update link in CONTRIBUTING.rst.

* Update to RST syntax.

* [Mega-Linter] Apply linters fixes

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Co-authored-by: umaannamalai <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: umaannamalai

CONTRIBUTING.rst

@@ -237,4 +237,5 @@ We host a public Slack with a dedicated channel for contributors and
 maintainers of open source projects hosted by New Relic. If you are
 contributing to this project, you're welcome to request access to the
 #oss-contributors channel in the newrelicusers.slack.com workspace. To
-request access, see https://newrelicusers-signup.herokuapp.com/.
+request access, please use this `link
+<https://join.slack.com/t/newrelicusers/shared_invite/zt-1ayj69rzm-~go~Eo1whIQGYnu3qi15ng/>`__.