Update sup_princ.xml

[neo-clj]

Of course, only one of them could be used: either "the" or "an".

But "one" is better than both of them.
https://ludwig.guru/s/important+exception

[Maria-12648430]

I see you opened 3 individual PRs, #7870, #7871 and this one, to address 3 minor documentation issues. Could you please wrap them up in 1 PR, even in 1 commit, and provide a better commit message (maybe "Fix typos in Supervision Principles docs" or something)?