Update collision-related comments

[Ghenohenomohe]

Description

This PR fixes function comments that are described in opposite.
Correct description should be collisionCallback() checks collision and distanceCallback() checks distance.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[welcome]

Thanks for helping in improving MoveIt and open source robotics!

[codecov]

Codecov Report

Merging #2382 into melodic-devel will decrease coverage by 0.05%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           melodic-devel    #2382      +/-   ##
=================================================
- Coverage          51.34%   51.29%   -0.04%     
=================================================
  Files                322      322              
  Lines              23142    23142              
=================================================
- Hits               11880    11869      -11     
- Misses             11262    11273      +11     

Impacted Files	Coverage Δ
.../moveit/collision_detection_fcl/collision_common.h	98.28% <ø> (ø)
.../ompl_interface/src/detail/constrained_sampler.cpp	42.86% <0.00%> (-17.14%)	⬇️
...nning_scene_monitor/src/planning_scene_monitor.cpp	66.94% <0.00%> (-0.47%)	⬇️
moveit_core/planning_scene/src/planning_scene.cpp	54.51% <0.00%> (-0.20%)	⬇️

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[felixvd]

Travis said "Expected — Waiting for status to be reported" but a younger build had passed, so I restarted the build.

[welcome]

Congrats on getting your first MoveIt pull request merged and improving open source robotics!

[felixvd]

Thanks for the PR! @Ghenohenomohe