Fix clippy::single-char-pattern

src/database/sending.rs

@@ -386,7 +386,7 @@ impl Sending {
         })?;
 
         // Appservices start with a plus
-        let (server, is_appservice) = if server.starts_with("+") {
+        let (server, is_appservice) = if server.starts_with('+') {
             (&server[1..], true)
         } else {
             (&*server, false)