docs(behavior_path_planner): fix object_check_min_road_shoulder_width explanation

[rej55]

Description

Explanation for object_check_min_road_shoulder_width is missed.
I add it.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (1b1ac5c) 15.10% compared to head (9b13bfb) 15.10%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4834      +/-   ##
==========================================
- Coverage   15.10%   15.10%   -0.01%     
==========================================
  Files        1573     1573              
  Lines      108318   108324       +6     
  Branches    33250    33253       +3     
==========================================
  Hits        16366    16366              
- Misses      74118    74124       +6     
  Partials    17834    17834              

Flag	Coverage Δ		*Carryforward flag
differential	13.01% <0.00%> (?)
total	15.10% <ø> (ø)		Carriedforward from 1b1ac5c

*This pull request uses carry forward flags. Click here to find out more.

Files Changed	Coverage Δ
...th_planner/scene_module/lane_change/base_class.hpp	15.55% <ø> (ø)
...r_path_planner/scene_module/lane_change/normal.hpp	0.00% <ø> (ø)
...planner/src/scene_module/lane_change/interface.cpp	6.30% <0.00%> (-0.06%)	⬇️
...th_planner/src/scene_module/lane_change/normal.cpp	4.05% <0.00%> (-0.03%)	⬇️

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