It's not bad.
You have prepared the correct Pull Request in your fork based on the branch named "pj7-patch-1".
In this case, you need to act "bolder" and include the Pull Request in the master documentation branch based on your branch.
This is done here:
https://github.com/Sigil-Ebook/sigil...de:pj7-patch-1
Note that the left is the main "master" branch of the documentation, and the right is your "pj7-patch-1" branch with changes. The direction of the arrow is important: it must lead from your branch to the main "master" branch.
Nobody starts out as an expert.