doc: remove linking of url text to url

Linking URL text to itself is superfluous. It will display as a link in
GitHub anyway. Bonus: This makes it possible to wrap the line at 80
characters.

PR-URL: #21361
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

doc/onboarding.md

@@ -206,7 +206,7 @@ needs to be pointed out separately during the onboarding.
 ## Exercise: Make a PR adding yourself to the README
 
 * Example:
-  [https://github.com/nodejs/node/commit/ce986de829457c39257cd205067602e765768fb0][]
+  https://github.com/nodejs/node/commit/ce986de829457c39257cd205067602e765768fb0
   * For raw commit message: `git log ce986de829457c39257cd205067602e765768fb0
     -1`
 * Collaborators are in alphabetical order by GitHub username.
@@ -250,7 +250,6 @@ needs to be pointed out separately during the onboarding.
 [`git-node`]: https://github.com/nodejs/node-core-utils/blob/master/docs/git-node.md
 [`node-core-utils`]: https://github.com/nodejs/node-core-utils
 [Landing Pull Requests]: https://github.com/nodejs/node/blob/master/COLLABORATOR_GUIDE.md#landing-pull-requests
-[https://github.com/nodejs/node/commit/ce986de829457c39257cd205067602e765768fb0]: https://github.com/nodejs/node/commit/ce986de829457c39257cd205067602e765768fb0
 [Publicizing or hiding organization membership]: https://help.github.com/articles/publicizing-or-hiding-organization-membership/
 [set up the credentials]: https://github.com/nodejs/node-core-utils#setting-up-credentials
 [two-factor authentication]: https://help.github.com/articles/securing-your-account-with-two-factor-authentication-2fa/