Function edit_sprint 

pub async fn edit_sprint(
    project_dir: &Path,
    id: &SprintId,
    title: Option<String>,
    goal: Option<String>,
    period: Option<(DateTime<Utc>, DateTime<Utc>)>,
) -> Result<Sprint>

Update the title, goal, and/or planned period of an existing sprint.

Fields set to None remain unchanged. Return Error::NothingToUpdate when no field is supplied, Error::EmptySprintTitle for a blank title, Error::InvalidSprintPeriod for an inverted period, or Error::SprintNotFound when the sprint does not exist.