Fix typo + small changes

Thanks to Lucas das Dores
https://stacks.math.columbia.edu/tag/022Y#comment-5569

groupoids.tex

@@ -1978,23 +1978,27 @@ \section{Actions of group schemes}
 In situation (1) this means that the diagrams
 \begin{equation}
 \label{equation-action}
+\vcenter{
 \xymatrix{
 G \times_S G \times_S X \ar[r]_-{1_G \times a} \ar[d]_{m \times 1_X} &
 G \times_S X \ar[d]^a \\
 G \times_S X \ar[r]^a & X
 }
-\quad
+}
+\quad\quad
+\vcenter{
 \xymatrix{
 G \times_S X \ar[r]_-a & X \\
 X\ar[u]^{e \times 1_X} \ar[ru]_{1_X}
 }
+}
 \end{equation}
 are commutative. In situation (2) this just means that the diagram
 $$
 \xymatrix{
-G \times_S X \ar[r]_-{\text{id} \times f} \ar[d]_a &
+G \times_S X \ar[r]_-{\text{id} \times \psi} \ar[d]_a &
 G \times_S Y \ar[d]^a \\
-X \ar[r]^f & Y
+X \ar[r]^\psi & Y
 }
 $$
 commutes.