doc: rename good first contrib label

Now `good first issue`.

PR-URL: #16150
Refs: #16149
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>

doc/onboarding-extras.md

@@ -63,7 +63,7 @@ Please use these when possible / appropriate
 * `confirmed-bug` - Bugs you have verified exist
 * `discuss` - Things that need larger discussion
 * `feature request` - Any issue that requests a new feature (usually not PRs)
-* `good first contribution` - Issues suitable for newcomers to process
+* `good first issue` - Issues suitable for newcomers to process
 
 --