Fix typo in stacks-introduction.tex

Thanks to Joseph Gunther
http://stacks.math.columbia.edu/tag/072Q#comment-1679

stacks-introduction.tex

@@ -328,7 +328,7 @@ \section{The definition}
 then this should be familiar. Anyway, the ``glueing'' condition on
 $\mathcal{M}_{1, 1}$ says that if you have a solution to this
 set of equations, then there exists an elliptic curve $E'$ over $S$
-such that $E \cong E \times_S T$ (it says a little bit more because
+such that $E \cong E' \times_S T$ (it says a little bit more because
 it also tells you how to recover the $\alpha_\sigma$).
 
 \medskip\noindent