chore[tool]: add help text for hex-ir CLI flag

[antazoey]

What I did

I noticed --hex-ir was missing a help.
Simple PR adds one.

side nit: I notice inconsistencies with CLI help texts... Some start with capital letter, some have periods at the end, etc.

How I did it

help=

How to verify it

vyper --help

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

Cute Animal Picture

woof

[charles-cooper]

seems fine. please fix lint!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.69%. Comparing base (cb94068) to head (196871c).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3942      +/-   ##
==========================================
- Coverage   90.85%   90.69%   -0.16%     
==========================================
  Files          95       95              
  Lines       14416    14449      +33     
  Branches     3192     3204      +12     
==========================================
+ Hits        13097    13105       +8     
- Misses        913      928      +15     
- Partials      406      416      +10     

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