services/horizon/internal: fix flag usage for max-concurrent-requests

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

We are using go-chi version 4.1 in horizon:

https://github.com/stellar/go/blob/release-horizon-v2.29.0/go.mod#L22

And, in that version, the request throttler middleware uses a response code of 503 when the number of in flight requests exceeds the limit:

https://github.com/go-chi/chi/blob/v4.1.3/middleware/throttle.go#L106

Known limitations

[N/A]