Correct description of "En" output for Priority Encoder

[TobiasKappe]

The "En" port was described as an input to the priority encoder, when really it is an output that indicates whether the input is not all zero (and hence the outputs Tn are not meaningful).

The circuit shown in the iframe also needs to be updated, but I cannot do that. Right now, it connects an input to the En port, which triggers a contention error.

Ref CircuitVerse#1718: This issue was spotted quite a while ago, but the proposed fix changed the name of the output port, and was rejected. This change merely documents the status quo, which I think is necessary - multiple students of mine were confused by the documentation!

Changes done:

• Update the documentation to match the actual behavior of the priority encoder (see above).

Screenshots:

Not applicable.

Preview Link(s):

https://github.com/TobiasKappe/CircuitVerseDocs/blob/897f909b1fef934241372fcca955d089a848846d/docs/chapter4/5muxandplex.md#priority-encoder

✅️ By submitting this PR, I have verified the following

• Checked to see if a similar PR has already been opened (there's Fix description of Priority Encoder Enable to 'Valid' output. #276, which is in conflict with master, and is not getting merged on account of the accompanying PR to CircuitVerse being rejected)
• Reviewed the contributing guidelines 🔍️
• Sample preview link added (add a link from the checks tab after checks complete)
• Tried Squashing the commits into one