docs(vehicle_velocity_converter): sentence correction in README

[IshitaTakeshi]

Related Issue(required)

Very small spell correction so no issue is required

Description(required)

Just replace This package is converted -> This package converts

Review Procedure(required)

Just look at it

Related PR(optional)

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PR Author should check the checkboxes below when creating the PR.

• Read commit-guidelines
• Assign PR to reviewer

If you are adding new package following items are required:

• Documentation with description of the package is available
• A sample launch file and parameter file are available if the package contains executable nodes

Checklist for the PR Reviewer

Reviewers should check the checkboxes below before approval.

• Commits are properly organized and messages are according to the guideline
• PR title describes the changes

Post-Review Checklist for the PR Author

PR Author should check the checkboxes below before merging.

• All open points are addressed and tracked via issues or tickets

CI Checks

• Build and test for PR / build-and-test-pr: Required to pass before the merge.
• Build and test for PR / clang-tidy-pr: NOT required to pass before the merge. It is up to the reviewer(s). Found false positives? See the [guidelines][clang-tidy-guidelines].
• Check spelling: NOT required to pass before the merge. It is up to the reviewer(s). See here if you want to add some words to the spell check dictionary.

[kenji-miyake]

Please change the title and commit message to follow https://www.conventionalcommits.org/.

[codecov]

Codecov Report

Merging #367 (bd2956f) into tier4/proposal (59f0f77) will decrease coverage by 9.29%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           tier4/proposal    #367      +/-   ##
=================================================
- Coverage            9.29%   0.00%   -9.30%     
=================================================
  Files                 700       3     -697     
  Lines               48932      25   -48907     
  Branches             5817       0    -5817     
=================================================
- Hits                 4549       0    -4549     
+ Misses              40572      25   -40547     
+ Partials             3811       0    -3811     

Impacted Files	Coverage Δ
...tem/system_monitor/src/mem_monitor/mem_monitor.cpp
...llower_nodes/test/test_lateral_controller_node.cpp
...terpolation/test/src/test_spline_interpolation.cpp
perception/tensorrt_yolo/lib/src/plugins/nms.cu
...planner/src/scene_module/traffic_light/manager.cpp
perception/tensorrt_yolo/src/nodelet.cpp
planning/motion_velocity_smoother/src/resample.cpp
...ult_injection/test/src/test_diagnostic_storage.cpp
.../src/scene_module/side_shift/side_shift_module.cpp
...include/ad_service_state_monitor/state_machine.hpp
... and 687 more

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