Delete @ on contextual help for Nickname when Signing Up

This message from signing up process: Nickname* [ ] Your alias (@nickname) in Metadecidim Makes user write their "alias" with an at, and this causes and error with no concrete error message of return. We had received many incidences from users that couldn't finish their registration process and these were solved when we told them not to use "at" at the begining of nickname. So in order to make message more clear, we thought its better not to write (@nickname) on the help text for nickname.