Fix equivalence relation defining the stalk

Thanks to Bas Edixhoven
http://stacks.math.columbia.edu/tag/0078#comment-281

Author: aisejohan

Diff for: sheaves.tex

@@ -923,10 +923,10 @@ \section{Stalks}
 x\in U, s\in \mathcal{F}(U)
 \}/\sim
 $$
-with equivalence relation given by $(U, s) \sim (U', s')$
-if and only if $s|_{U\cap U'} = s'|_{U\cap U'}$. By abuse of
-notation we will often denote $(U, s)$, $s_x$,
-or even $s$ the corresponding element in $\mathcal{F}_x$.
+with equivalence relation given by $(U, s) \sim (U', s')$ if and only if
+there exists an open $U'' \subset U \cap U'$ with $x \in U''$ and
+$s|_{U''} = s'|_{U''}$. By abuse of notation we will often denote
+$(U, s)$, $s_x$, or even $s$ the corresponding element in $\mathcal{F}_x$.
 Also we will say $s = s'$ in $\mathcal{F}_x$ for two local sections
 of $\mathcal{F}$ defined in an open neighbourhood of $x$ to denote that
 they have the same image in $\mathcal{F}_x$.