Fix typo: simlar → similar in troubleshooting docs

Bob Singhrequested to merge
gitlab-community/gitlab-org/gitlab-runner:fix-simlar-typo into main

What does this MR do? Fixes a misspelling (simlar → similar).

Why was this MR needed? To correct a typo and improve documentation clarity.

How to test? Open the troubleshooting docs and verify the corrected word appears properly.

What are the relevant issue numbers?

N/A

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