fix one other subscript

categories.tex

@@ -952,7 +952,7 @@ \section{The Yoneda lemma}
   This gives a function $(\alpha \mapsto \alpha_a(1_a))$ from left to right in~\eqref{eq:yoneda}.
 
   In the other direction, given $x:F a$, we define $\alpha:\y a \to F$ by
-  \[\alpha_{a'}(f) \defeq F_{a',a}(f)(x). \]
+  \[\alpha_{a'}(f) \defeq F_{a,a'}(f)(x). \]
   Naturality is easy to check, so this gives a function from right to left in~\eqref{eq:yoneda}.
 
   To show that these are inverses, first suppose given $x:F a$.