Remove a

Thanks to Wessel Bindt
https://stacks.math.columbia.edu/tag/054Q#comment-4583

varieties.tex

@@ -1420,7 +1420,7 @@ \section{Geometrically irreducible schemes}
 \begin{lemma}
 \label{lemma-geometrically-irreducible-function-field}
 Let $X$ be a geometrically irreducible scheme over the field $k$.
-Let $\xi \in X$ be its generic point. Then $\kappa(\xi)$ is a
+Let $\xi \in X$ be its generic point. Then $\kappa(\xi)$ is
 geometrically irreducible over $k$.
 \end{lemma}