docs: polish pitr README.md

[mlycore]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

fix: #440

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov-commenter]

Codecov Report

Merging #441 (78d680f) into main (3604a24) will decrease coverage by 3.40%.
Report is 177 commits behind head on main.
The diff coverage is 27.40%.

@@            Coverage Diff             @@
##             main     #441      +/-   ##
==========================================
- Coverage   39.71%   36.32%   -3.40%     
==========================================
  Files          51       59       +8     
  Lines        6083     6615     +532     
==========================================
- Hits         2416     2403      -13     
- Misses       3448     3985     +537     
- Partials      219      227       +8     

Files Changed	Coverage Δ
pitr/agent/internal/handler/restore.go	0.00% <0.00%> (ø)
pitr/cli/internal/cmd/restore.go	58.33% <0.00%> (-0.99%)	⬇️
pitr/cli/internal/pkg/local-storage.go	40.65% <0.00%> (-2.09%)	⬇️
...tor/cmd/shardingsphere-operator/manager/manager.go	0.00% <0.00%> (ø)
...operator/pkg/controllers/auto_scaler_controller.go	0.00% <0.00%> (ø)
...phere-operator/pkg/controllers/chaos_controller.go	0.00% <0.00%> (ø)
...perator/pkg/controllers/compute_node_controller.go	0.00% <0.00%> (ø)
...rdingsphere-operator/pkg/kubernetes/pod/builder.go	0.00% <0.00%> (ø)
...hardingsphere-operator/pkg/kubernetes/resources.go	0.00% <0.00%> (ø)
...gsphere-operator/pkg/kubernetes/service/builder.go	76.92% <0.00%> (ø)
... and 15 more

... and 2 files with indirect coverage changes

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