config: Bring "unique... within this map" back together

Pull the empty-key restriction out into its own sentence (instead of
leaving it in the middle of the uniqueness restriction).

Drop the "best practice" portion, because the following line is "Keys
SHOULD be named using a reverse domain notation...", which covers that
idea more explicitly.

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -393,7 +393,8 @@ If a hook returns a non-zero exit code, then an error is logged and the remainin
 This information MAY be structured or unstructured.
 Annotations MUST be a key-value map where both the key and value MUST be strings.
 While the value MUST be present, it MAY be an empty string.
-Keys MUST be unique and MUST NOT be an empty string within this map, and best practice is to namespace the keys.
+Keys MUST be unique within this map.
+Keys MUST NOT be an empty string.
 Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
 Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by subsequent specifications.
 If there are no annotations then this property MAY either be absent or an empty map.