fixed key on midWild use case on config tests

[neeta-rathi]

Mid wild card on descriptor values is not supported by ratelimit service. However the test around this use case ( on test/config/config_test.go ) fails because the key on the RateLimitDescriptor is midWildcard instead of midWild as it's defined on the wildcard.yaml https://github.com/envoyproxy/ratelimit/blob/main/test/config/wildcard.yaml#L18

On this PR I'm changing the key to midWild so it matches to how it's defined on the wildcard.yaml and now the test fails rightfully for not supporting the mid wildcards.

cc: @envoyproxy/ratelimit-maintainers