making working clearer

*should* implies that an `extra` can be mixed case.  but really it can't because a mixed case `extra` will mismatch on an RBAC `ClusterRole` once the header is canonicalized.

Author: mlbiam

content/en/docs/reference/access-authn-authz/authentication.md

@@ -733,7 +733,7 @@ The following HTTP headers can be used to performing an impersonation request:
 
 * `Impersonate-User`: The username to act as.
 * `Impersonate-Group`: A group name to act as. Can be provided multiple times to set multiple groups. Optional. Requires "Impersonate-User".
-* `Impersonate-Extra-( extra name )`: A dynamic header used to associate extra fields with the user. Optional. Requires "Impersonate-User". In order to be preserved consistently, `( extra name )` should be lower-case, and any characters which aren't [legal in HTTP header labels](https://tools.ietf.org/html/rfc7230#section-3.2.6) MUST be utf8 and [percent-encoded](https://tools.ietf.org/html/rfc3986#section-2.1).
+* `Impersonate-Extra-( extra name )`: A dynamic header used to associate extra fields with the user. Optional. Requires "Impersonate-User". In order to be preserved consistently, `( extra name )` must be lower-case, and any characters which aren't [legal in HTTP header labels](https://tools.ietf.org/html/rfc7230#section-3.2.6) MUST be utf8 and [percent-encoded](https://tools.ietf.org/html/rfc3986#section-2.1).
 * `Impersonate-Uid`: A unique identifier that represents the user being impersonated. Optional. Requires "Impersonate-User". Kubernetes does not impose any format requirements on this string.
 
 {{< note >}}