Fix link of installation tutorial to point to 7 version

[traversaro]

🦟 Bug fix

While navigating with @giotherobot we noticed that the link was pointing to the 6.8 version. Not sure if there is a smarted fix, link linking to latest or similar.

Summary

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 #518 (d18a5fb) into gz-math7 (a2f014e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           gz-math7     #518   +/-   ##
=========================================
  Coverage     99.70%   99.70%           
=========================================
  Files            77       77           
  Lines          7026     7026           
=========================================
  Hits           7005     7005           
  Misses           21       21           

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