fix: update outdated error doc (#699)

Author: mrousavy

Sources/Auth/AuthClient.swift

@@ -56,7 +56,7 @@ public actor AuthClient {
 
   /// Returns the session, refreshing it if necessary.
   ///
-  /// If no session can be found, a ``AuthError/sessionNotFound`` error is thrown.
+  /// If no session can be found, a ``AuthError/sessionMissing`` error is thrown.
   public var session: Session {
     get async throws {
       try await sessionManager.session()