Fix nits in PingSource v1beta2

[eclipselu]

Follow-up to #4474

Proposed Changes

• 🐛 Fixes unaddressed nits in add PingSource v1beta2 #4474

Release Note

Docs

[eclipselu]

/assign @nachocano

[codecov]

Codecov Report

Merging #4537 (039cafd) into master (3fca4a2) will decrease coverage by 0.10%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #4537      +/-   ##
==========================================
- Coverage   81.42%   81.31%   -0.11%     
==========================================
  Files         290      290              
  Lines        8150     8157       +7     
==========================================
- Hits         6636     6633       -3     
- Misses       1121     1126       +5     
- Partials      393      398       +5     

Impacted Files	Coverage Δ
pkg/apis/sources/v1beta2/ping_types.go	100.00% <ø> (ø)
pkg/adapter/mtping/runner.go	84.48% <25.00%> (-9.97%)	⬇️
pkg/apis/sources/v1beta1/ping_conversion.go	89.09% <33.33%> (-6.99%)	⬇️
pkg/apis/sources/v1beta2/ping_validation.go	100.00% <100.00%> (ø)

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[nachocano]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: eclipselu, nachocano

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/adapter/mtping/runner.go	97.8%	91.8%	-5.9
pkg/apis/sources/v1beta1/ping_conversion.go	97.7%	93.8%	-4.0