Update url-strategies.md

[chrismercredi]

You can't add flutter_web_plugins directly using pub add. Its only included as part of the project if required by other dependencies like go_router.

Description of what this PR is changing or adding, and why:

Added additional information on how to add flutter_web_plugins to project to use usePathUrlStrategy() function.

Issues fixed by this PR (if any):

You can't add flutter_web_plugins directly using pub add or other methods.

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

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[sfshaza2]

lgtm