Fixing duplicate id.

Author: Sanders Kleinfeld

book/06-github/sections/2-contributing.asc

@@ -242,7 +242,7 @@ To https://github.com/tonychacon/blink
 
 Once you do that, the Pull Request will be automatically updated and re-checked to see if it merges cleanly.
 
-[[_pr_fail]]
+[[_pr_merge_fix]]
 .Pull Request now merges cleanly
 image::images/pr-02-merge-fix.png[PR fixed]