Small fixes to the READMEs and CLI help messages

[haochenuw]

In this PR I

• updated the README in the example directory to include the MySQL docker instance spin-up command.
• updated the help message for the CLI in mysql_demo. Since it is no longer supported to get root_hash of an older epoch, I modified the description of root_hash command to remove the optional epoch parameter from the CLI help message.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.44%. Comparing base (24b11de) to head (1d22b5a).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
+ Coverage   87.99%   88.44%   +0.44%     
==========================================
  Files          39       39              
  Lines        9054     9214     +160     
==========================================
+ Hits         7967     8149     +182     
+ Misses       1087     1065      -22     

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[kevinlewi]

Looks good, thanks! I know it's unrelated, but mind addressing the clippy error in this diff as well?

[kevinlewi]

LGTM