Render usage as text instead of bash

[jtraglia]

📝 Summary

Minor change. In the README, the usage (mev-boost -help) should be rendered as text instead of bash.

⛱ Motivation and Context

Some of the words (keywords) had syntax highlighting. I found it a bit distracting:

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

Codecov Report

Merging #388 (999bf5e) into main (eacfd5b) will increase coverage by 0.11%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
+ Coverage   66.66%   66.77%   +0.11%     
==========================================
  Files           7        7              
  Lines         888      888              
==========================================
+ Hits          592      593       +1     
+ Misses        262      259       -3     
- Partials       34       36       +2     

Flag	Coverage Δ
unittests	66.77% <0.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cli/main.go	13.51% <0.00%> (ø)
server/mock_relay.go	86.92% <ø> (ø)
server/service.go	79.52% <0.00%> (+0.23%)	⬆️

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