Update docs/adding_software/debugging_failed_builds.md

Co-authored-by: Kenneth Hoste <kenneth.hoste@ugent.be>

Author: TopRichard

docs/adding_software/debugging_failed_builds.md

@@ -146,10 +146,11 @@ Note that you could also combine approaches: first build everything using the `E
 
 To build software using the `EESSI-install-software.sh` script, you'll first need to get the diff file for the PR. This is used by the `EESSI-install-software.sh` script to see what is changed in this PR - and thus what needs to be build for this PR. To download the diff for PR 360, we would e.g. do
 ```
-wget https://github.com/EESSI/software-layer/pull/360.diff
+PR=123456789    # change this to correct the PR number
+wget https://github.com/EESSI/software-layer/pull/${PR}.diff
 git clone https://github.com/EESSI/software-layer.git
 cd software-layer
-git pull origin pull/360/head
+git pull origin pull/${PR}/head
 cd ..
 cp -r software-layer/easystacks .
 ```