doc: fix use of old and confusing router terminology

[rfdonnelly]

Description

Replace old "router" terminology with "Coordinator". This seems to have been missed by e0167ae.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.6%. Comparing base (0b3babf) to head (164152d).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1716   +/-   ##
======================================
  Coverage    55.6%   55.6%           
======================================
  Files         172     172           
  Lines       13495   13495           
======================================
  Hits         7514    7514           
  Misses       5981    5981           

Flag	Coverage Δ
3.10	55.6% <ø> (ø)
3.11	55.6% <ø> (ø)
3.12	55.6% <ø> (ø)
3.13	55.6% <ø> (ø)
3.9	55.7% <ø> (ø)

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[jluebbe]

Thanks for this PR! Could you add a Signed-off-by? see https://github.com/labgrid-project/labgrid/pull/1716/checks?check_run_id=48698569068

[rfdonnelly]

Added signed-off-by.