Fix base64 encoding example for URL quoting (3.1.1 port of 2923 5/6)

I forgot to run the example through URL quoting, which is necessary.

Author: handrews

versions/3.1.1.md

@@ -1796,9 +1796,13 @@ Given a name of `example` and a solid red 2x2-pixel PNG for `icon`, this
 would produce a request body of:
 
 ```urlencoded
-name=example&icon=iVBORw0KGgoAAAANSUhEUgAAAAIAAAACCAYAAABytg0kAAAAEklEQVQIW2P8z8AARAwMjDAGACwBA/+8RVWvAAAAAElFTk
+name=example&icon=iVBORw0KGgoAAAANSUhEUgAAAAIAAAACCAIAAAD91JpzAAAABGdBTUEAALGPC_xhBQAAADhlWElmTU0AKgAAAAgAAYdpAAQAAAABAAAAGgAAAAAAAqACAAQAAAABAAAAAqADAAQAAAABAAAAAgAAAADO0J6QAAAAEElEQVQIHWP8zwACTGCSAQANHQEDqtPptQAAAABJRU5ErkJggg%3D%3D
 ```
 
+Note that the `=` padding characters at the end need to be percent-encoded, even with the "URL safe" `contentEncoding: base64url`.
+Some base64-decoding implementations may be able to use the string without the padding per [RFC4648 §3.2](https://datatracker.ietf.org/doc/html/rfc4648#section-3.2).
+However, this is not guaranteed, so it may be more interoperable to keep the padding and rely on percent-decoding.
+
 ##### Encoding `multipart` Media Types
 
 It is common to use `multipart/form-data` as a `Content-Type` when transferring request bodies to operations.  In contrast to 2.0, a `schema` is REQUIRED to define the input parameters to the operation when using `multipart` content.  This supports complex structures as well as supporting mechanisms for multiple file uploads.