Fix stray "format: byte" to use contentEncoding

Author: handrews

versions/3.1.1.md

@@ -1679,9 +1679,10 @@ requestBody:
           name:
             type: string
           icon:
-            # default is text/plain, need to declare an image type only!
+            # default for type string is text/plain, need to declare
+            # the appropriate contentType in the Encoding Object
             type: string
-            format: byte
+            contentEncoding: base64url
   encoding:
     icon:
       contentType: image/png, image/jpeg