[nits] fix docs to lowerCamelCase and add t.Parallel()

[t-kikuc]

What this PR does / why we need it:

• [docs] Fix field names to lowerCamelCase. https://github.com/pipe-cd/pipecd/pull/5008#discussion_r1719327610
• [nits] add t.Parallel() to a test case. https://github.com/pipe-cd/pipecd/pull/5008#discussion_r1719329240

Which issue(s) this PR fixes:

Related to #5008

Does this PR introduce a user-facing change?: No

• How are users affected by this change: N/A
• Is this breaking change: no
• How to migrate (if breaking change): N/A

[codecov[bot]]

Codecov Report

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

Project coverage is 22.89%. Comparing base (a61c397) to head (f6c8a63). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5173 +/- ## ========================================== - Coverage 22.90% 22.89% -0.01% ========================================== Files 416 416 Lines 44598 44598 ========================================== - Hits 10216 10212 -4 - Misses 33592 33595 +3 - Partials 790 791 +1 ```

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