Fix table in "Nodes and topics tutorial"

[caguero]

🦟 Bug fix

Summary

The tutorial "Nodes and Topics" uses two tables to show legal/ilegal topics and namespaces. The empty topics and namespaces are rendered as </td>. I'm trying this fix but not 100% sure if this will fix it.

Tagging @nkoenig in case he has extra tips.

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 #356 (48a3608) into gz-transport12 (a31c20f) will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           gz-transport12     #356   +/-   ##
===============================================
  Coverage           88.11%   88.11%           
===============================================
  Files                  52       52           
  Lines                4848     4848           
===============================================
  Hits                 4272     4272           
  Misses                576      576           

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[ahcorde]

sign commit