Fix undefined ErrSessionExpired

Fixes: f07ab52632 ("Add docs")

server.go

@@ -176,7 +176,7 @@ func New(e *echo.Echo, options *Options) error {
 			}
 
 			ctx.Session, err = ctx.Server.Sessions.get(cookie.Value)
-			if err == ErrSessionExpired {
+			if err == errSessionExpired {
 				ctx.SetSession(nil)
 				return ctx.Redirect(http.StatusFound, "/login")
 			} else if err != nil {

session.go

@@ -115,7 +115,7 @@ func (sm *SessionManager) get(token string) (*Session, error) {
 
 	session, ok := sm.sessions[token]
 	if !ok {
-		return nil, ErrSessionExpired
+		return nil, errSessionExpired
 	}
 	return session, nil
 }