Update description for DAST API scan file path option

Related discussion - !79279 (comment 1081631756)

What does this MR do and why?

This MR updates placeholder and description text for scan file path option in DAST site profile.

This is because with this first iteration, we want to support only the URL instead of file paths.

Screenshots or screen recordings

See the proposal with before / after

Updated version

before	after

option 1	option 2	option 3

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:dast_api_scanner)

2. Visit /security/configuration/profile_library/dast_site_profiles/new to create a new site profile
3. Select API option

MR acceptance checklist

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