Fixes https://github.com/matrix-org/matrix-doc/issues/2066
The expectation everywhere is that the `sender` is required. `/initialSync` references StrippedState through a `m.room.member` event reference, and does not need editing.
# is reserved by the swagger validator as a way to include partial content from a JSON object (eg: "#/path" would include {"test": true} from the object {"path":{"test":true}}). Instead of trying to convince the validator that it is wrong, we'll just use a different character.
Note that our rendering tools do not care about #-style references to objects. It's still somewhat worth changing the character though.
Travis Ralston