Improve error when a table is passed for a single key

I attempted to use a Vector config that included the new encoding table but it was on an older version of Vector. It produced the following configuration error:

[sinks.company_api_http]
type = "http"
inputs = ["company_api_rename"]
uri = "http://{{ consumer_instance.ec2_private_ip_address }}:{{ consumer_port }}"

[sinks.company_api_http.encoding]
except_fields = ["metadata.shard[0]", "metadata.shard[1]"]

The valid config is:

[sinks.company_api_http]
type = "http"
inputs = ["company_api_rename"]
uri = "http://{{ consumer_instance.ec2_private_ip_address }}:{{ consumer_port }}"
encoding.format = "json"

Mar 14 17:39:21 ip-10-0-131-11 vector[11926]: Mar 14 17:39:21.233 ERROR vector: Configuration error: "/etc/vector/vector.toml": invalid value: map, expected map with a single key for key `sinks.company_api_http`

This is not very descriptive of the error. It should probably be something like:

Mar 14 17:39:21 ip-10-0-131-11 vector[11926]: Mar 14 17:39:21.233 ERROR vector: Configuration error: "/etc/vector/vector.toml": invalid value: map, expected map with a single key for encoding `sinks.company_api_http`

Reproduce the configuration error with the shown sinks.company_api_http.encoding table on an older Vector version, then trace the configuration parsing path that emits the single-key map message. Update the error so it identifies encoding, and verify the revised wording against the provided configuration example.