docs: minor improvement for docs

[huyejia]

Types of changes

What types of changes does your code introduce to OpenZeppelin Midnight Contracts?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

minor improvement for docs

PR Checklist

• I have read the Contributing Guide
• I have added tests that prove my fix is effective or that my feature works
• I have added documentation of new methods and any new behavior or changes to existing behavior
• CI Workflows Are Passing

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[huyejia]

@0xisk @emnul @son-oz Hi, Could you please review this PR at your convenience? Thank you very much.