use variant instead of struct variant

Per Watson Ladd's suggestion at IETF 120 PPM session.

draft-ietf-ppm-dap.md

@@ -666,10 +666,10 @@ struct {
 ~~~
 
 Then we describe the specific variant of `ExampleStruct` where `type == number`
-with a `struct variant` block like so:
+with a `variant` block like so:
 
 ~~~
-struct variant {
+variant {
   /* Field exists regardless of variant */
   uint32 always_present;
   ExampleEnum type = number;
@@ -701,7 +701,7 @@ struct {
 The protocol text might include a description like:
 
 ~~~
-struct variant {
+variant {
   FailureReason failure_reason = something;
   opaque another_field<0..256>;
 } FailedOperation;
@@ -1564,7 +1564,7 @@ For any report that was rejected, the Helper sets the outbound preparation
 response to
 
 ~~~
-struct variant {
+variant {
   ReportID report_id;
   PrepareRespState prepare_resp_state = reject;
   PrepareError prepare_error;
@@ -1597,7 +1597,7 @@ initial `outbound` message to send in response to the Leader. If `state` is of
 type `Rejected`, then the Helper responds with
 
 ~~~
-struct variant {
+variant {
   ReportID report_id;
   PrepareRespState prepare_resp_state = reject;
   PrepareError prepare_error = vdaf_prep_error;
@@ -1607,7 +1607,7 @@ struct variant {
 Otherwise the Helper responds with
 
 ~~~
-struct variant {
+variant {
   ReportID report_id;
   PrepareRespState prepare_resp_state = continue;
   opaque payload<0..2^32-1> = outbound;
@@ -1883,7 +1883,7 @@ denote the payload of the prep step. The Helper computes the following:
 If `state == Rejected()`, then the Helper's response is
 
 ~~~
-struct variant {
+variant {
   ReportID report_id;
   PrepareRespState prepare_resp_state = reject;
   PrepareError prepare_error = vdaf_prep_error;
@@ -1893,7 +1893,7 @@ struct variant {
 Otherwise, if `outbound != None`, then the Helper's response is
 
 ~~~
-struct variant {
+variant {
   ReportID report_id;
   PrepareRespState prepare_resp_state = continue;
   opaque payload<0..2^32-1> = outbound;
@@ -1903,7 +1903,7 @@ struct variant {
 Otherwise, if `outbound == None`, then the Helper's response is
 
 ~~~
-struct variant {
+variant {
   ReportID report_id;
   PrepareRespState prepare_resp_state = finished;
 } PrepareResp;