s/NotAllowedError/TypeError for invalid rid & reference RFC8851. (#193)

* s/NotAllowedError/TypeError for invalid rid & reference RFC8851.

* Indent by 1 space to fix problem

index.bs

@@ -617,7 +617,9 @@ The <dfn attribute for="RTCRtpScriptTransformer">writable</dfn> getter steps are
 The <dfn abstract-op>generate key frame algorithm</dfn>, given |promise|, |encoder| and |rid|, is defined by running these steps:
 1. If |encoder| is undefined, reject |promise| with {{InvalidStateError}}, abort these steps.
 1. If |encoder| is not processing video frames, reject |promise| with {{InvalidStateError}}, abort these steps.
-1. If |rid| is defined, validate its value. If invalid, reject |promise| with {{NotAllowedError}} and abort these steps.
+1. If |rid| is defined, but does not conform to the grammar requirements specified
+    in Section 10 of [[!RFC8851]], then reject |promise| with {{TypeError}} and abort
+    these steps.
 1. [=In parallel=], run the following steps:
     1. Gather a list of video encoders, named |videoEncoders| from |encoder|, ordered according negotiated RIDs if any.
     1. If |rid| is defined, remove from |videoEncoders| any video encoder that does not match |rid|.