Add an internal reference to a proof

Thanks to Dario Weissmann
https://stacks.math.columbia.edu/tag/049W#comment-5972

groupoids-quotients.tex

@@ -389,7 +389,8 @@ \section{Categorical quotients}
 \begin{proof}
 Let $X_{red}$ be the reduction of the algebraic space $X$.
 Since $U$ is reduced the morphism $\phi : U \to X$ factors through
-$i : X_{red} \to X$ (insert future reference here). Denote this morphism
+$i : X_{red} \to X$ (Properties of Spaces, Lemma
+\ref{spaces-properties-lemma-map-into-reduction}). Denote this morphism
 by $\phi_{red} : U \to X_{red}$. Since $\phi \circ s = \phi \circ t$ we
 see that also $\phi_{red} \circ s = \phi_{red} \circ t$ (as
 $i : X_{red} \to X$ is a monomorphism). Hence by the universal property