Fix a minor issue in the documentation of the server API

[arjo129]

🦟 Bug fix

Fixes an inconsistency in the server API documentation where types of services aren't correctly named.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #2067 (dd532ff) into ign-gazebo3 (4587827) will not change coverage.
The diff coverage is n/a.

❗ Current head dd532ff differs from pull request most recent head 486c230. Consider uploading reports for the commit 486c230 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo3    #2067   +/-   ##
============================================
  Coverage        70.77%   70.77%           
============================================
  Files              262      262           
  Lines            16763    16763           
============================================
  Hits             11864    11864           
  Misses            4899     4899           

Files Changed	Coverage Δ
include/gz/sim/Server.hh	100.00% <ø> (ø)