Improve a remark in categories

Thanks to Manuel Hoff
https://stacks.math.columbia.edu/tag/003J#comment-5392

categories.tex

@@ -4945,8 +4945,8 @@ \section{2-categories}
 \begin{enumerate}
 \item The $2$-category of categories $\textit{Cat}$.
 \item The $(2, 1)$-category of categories $\textit{Cat}$.
-\item The $2$-category of groupoids $\textit{Groupoids}$.
-\item The $(2, 1)$-category of groupoids $\textit{Groupoids}$.
+\item The $2$-category of groupoids $\textit{Groupoids}$;
+this is a $(2, 1)$-category.
 \item The $2$-category of fibred categories over a fixed category.
 \item The $(2, 1)$-category of fibred categories over a fixed category.
 \end{enumerate}