Fix typo and add internal reference

Thanks to Dongryul Kim
https://stacks.math.columbia.edu/tag/08XA#comment-8763

descent.tex

@@ -1305,14 +1305,14 @@ \subsection{Descent for modules and their morphisms}
 Hence $f^*$ is not fully faithful.
 
 \medskip\noindent
-We finally check that (c) implies (b). By Lemma \ref{lemma-descent-lemma}, for
+We finally check that (c) implies (b). By Lemmas 
+\ref{lemma-equalizer-M} and \ref{lemma-descent-lemma}, for
 $(M, \theta) \in DD_{S/R}$,
 the natural map $f^* f_*(M,\theta) \to M$ is an isomorphism of $S$-modules. On 
 the other hand, for $M_0 \in \text{Mod}_R$,
 we may tensor (\ref{equation-equalizer-S}) with $M_0$ over $R$ to obtain an 
-equalizer sequence,
-so $M_0 \to f_* f^* M$ is an isomorphism. Consequently, $f_*$ and $f^*$ are 
-quasi-inverse functors, proving the claim.
+equalizer sequence, so $M_0 \to f_* f^* M_0$ is an isomorphism.
+Consequently, $f_*$ and $f^*$ are  quasi-inverse functors, proving the claim.
 \end{proof}
 
 \subsection{Descent for properties of modules}