Update PROTOCOL-WEB.md

doc/PROTOCOL-WEB.md

@@ -39,7 +39,7 @@ Content-Type
   * e.g. application/grpc-web+[proto, json, thrift]
 2. application/grpc-web-text
   * text-encoded streams of “application/grpc-web”
-  * e.g. application/grpc-web+[proto, thrift]
+  * e.g. application/grpc-web-text+[proto, thrift]
 
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