Fix broken external links in operations/feature_flags.md

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Documentation issue

The following files have broken links or redirects. Fix the links as follows:

In the file operations/feature_flags.md

[ ERROR ] external_links - broken reference to https://docs.getunleash.io/sdks/unleash-proxy: link has moved permanently to 'https://docs.getunleash.io/sdks/unleash-proxy/'
[ ERROR ] external_links - broken reference to https://docs.getunleash.io/user_guide/activation_strategy: link has moved permanently to 'https://docs.getunleash.io/user_guide/activation_strategy/'