Trivial issue with Documentation heading in Readme

[arjo129]

🦟 Bug fix

Currently the Documentation heading (and link) is broken in our readme. Probably due to some weird unicode issue where the space character is an impostor. This PR fixes it.

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 #1501 (c3112a1) into ign-gazebo6 (f4198be) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gazebo6    #1501   +/-   ##
============================================
  Coverage        33.58%   33.58%           
============================================
  Files               44       44           
  Lines             2260     2260           
============================================
  Hits               759      759           
  Misses            1501     1501           

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