Fix example link in SAST documentation

What does this MR do and why?

This merge request fix the SAST report example link in SAST documentation, which I found pointing to a secret detection example report file instead.

Please note the format is similar between gl-sast-report and gl-secret-detection-report files, but I thought it would be less confusing for users if we use an actual gl-sast-report file as the example, so I choose a dummy one from the semgrep analyzer project.

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