Update README.md to make long vs short flags clearer

[DanielTsiang]

Update README.md to make use of long vs short flags clearer, and fix copy-paste typo in the bottom table for ignoring notebook metadata.

I mistakenly just copy and pasted the command from the table but then was confused why it wasn't working...better to make it more foolproof 🙂

This PR addresses the following issue:

• https://github.com/srstevenson/nb-clean/issues/252

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (80640b4) 99.46% compared to head (740a78f) 99.46%.

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```diff @@ Coverage Diff @@ ## main #253 +/- ## ======================================= Coverage 99.46% 99.46% ======================================= Files 3 3 Lines 187 187 ======================================= Hits 186 186 Misses 1 1 ```

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[DanielTsiang]

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@srstevenson thanks for looking at my PR, could you help merge it please?

[DanielTsiang]

@srstevenson That makes sense, thanks for explaining!

From your reply on the issue here, I only just realised that the flag can go before or after the notebook filename (but has to go after clean or check). It wasn't obvious to me from the docs.

I personally think this:

$ nb-clean check --preserve-notebook-metadata my-notebook.ipynb

is a lot neater/simpler than this (which is how it's written in the docs), especially when using it in the CI and you're passing filenames to the check or clean command via xargs:

$ nb-clean check my-notebook.ipynb --preserve-notebook-metadata