updated the text to fit PR as a variable

Author: Richard Top

docs/adding_software/debugging_failed_builds.md

@@ -144,7 +144,9 @@ The Automatic build and deploy [bot](../bot.md) installs software by executing t
 
 Note that you could also combine approaches: first build everything using the `EESSI-install-software.sh` script, until you reproduce the failure. Then, start making modifications (e.g. changes to the EasyConfig, patches, etc) and trying to rebuild that package individually to test your changes.
 
-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
+To build software using the EESSI-install-software.sh script, you first need to download the diff file for the relevant pull request. The script uses this diff to determine which changes are included in the PR and therefore which software needs to be built.
+
+We now define the PR number through a ${PR} variable and use it throughout the commands. For example:
 ```
 PR=123456789    # change this to correct the PR number
 wget https://github.com/EESSI/software-layer/pull/${PR}.diff