Fix typo in "Server Behavior" section (#633)

A shared mode server should never recieve a "inner" type
ECHClientHello.type ClientHello, rather than an "outer" type, not the
inverse.

See https://mailarchive.ietf.org/arch/msg/tls/d00rdlmaVbnfDsvVnF_6pWobQQQ/.

draft-ietf-tls-esni.md

@@ -1008,7 +1008,7 @@ MUST abort with an "illegal_parameter" alert.
 In shared mode, a server plays both roles, first decrypting the
 `ClientHelloOuter` and then using the contents of the
 `ClientHelloInner`.  A shared mode server which receives a
-`ClientHello` with `ECHClientHello.type` of `outer` MUST abort with an
+`ClientHello` with `ECHClientHello.type` of `inner` MUST abort with an
 "illegal_parameter" alert, because such a `ClientHello` should never
 be received directly from the network.