fix(documentation): Fix the RST links to GHC.Prim

This MR fixes some inconsistencies that were spotted in the GHC User’s Guide. The custom RST link syntax was broken when mentionning GHC.Prim:

With this fix, it now looks like this:

⚠ If you know of other places where I could apply such syntactic fixes, please let me know, I will gladly integrate them in this MR!

You can see it live here

cc @core-libraries / @carter / @sjakobi
Tracking: #17929