Update user.go

[skam22]

Fixed typo in --user-id

Updated set flag to required since an unset command without a set command fails with Error: unexpected end of JSON input.

example:

stream-cli chat update-user-partial --user-id SAMPLE_USER_ID -u username

outputs:

Error: unexpected end of JSON input

stream-cli chat update-user-partial --user-id SAMPLE_USER_ID --set {} -u username

outputs:

Successfully updated user [SAMPLE_USER_ID]