Improved link in modsecurity.md

[borekb]

What this PR does / why we need it: small fix in a Markdown link (URL portion was not correct).

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): no issue

Special notes for your reviewer: I've also updated the link text slightly to make it more readable, hope it's OK.

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[borekb]

CLA signed.

[aledbf]

/lgtm
/approve

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This pull-request has been approved by: aledbf, borekb

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[aledbf]

@borekb thanks!