Add period

Thanks to Peng Du
https://stacks.math.columbia.edu/tag/0FL2#comment-7807

discriminant.tex

@@ -2200,7 +2200,7 @@ \section{Finite syntomic morphisms}
 With notation as in Example \ref{example-universal-finite-syntomic}
 consider the open subscheme $U'_d \subset X_d$ over which
 $\pi_d$ is \'etale. Then $U'_d$ is a dense subset of the
-open $U_d$ of Lemma \ref{lemma-universal-finite-syntomic}
+open $U_d$ of Lemma \ref{lemma-universal-finite-syntomic}.
 \end{lemma}
 
 \begin{proof}