error tikz graph reference (??)

Question

I don't know why appear this in my code

This is my code

\documentclass[12pt, openany]{book}
\usepackage{pgfplots}
\usepackage{tikz-3dplot}
\begin{ejemplo}

Hallemos los extremos de  $(x^2+y^2)e^{-x^2-y^2}$. véase figura \ref{parw}


\begin{figure}[htb]
\centering
\begin{tikzpicture}
    \begin{axis}[
        width=0.5\linewidth,
        xlabel=$x$,ylabel=$y$
        ]
        \addplot3[surf,domain=-2:2,domain y=-2:2]
        {((x^2+y^2)*exp(-x^2-y^2))};
    \end{axis}
\end{tikzpicture}
\label{parw}
\caption{}
\end{figure}

\end{ejemplo}
\end{document}