group act -> group operation in grp-intro

tex/H113/grp-intro.tex

@@ -380,7 +380,7 @@ \section{Isomorphisms}
 
 Specifically the map
 \[ \phi : \ZZ \to 10 \ZZ  \text{ by } x \mapsto 10 x \]
-is a bijection of the underlying sets which respects the group action.
+is a bijection of the underlying sets which respects the group operation.
 In symbols,
 \[ \phi(x + y) = \phi(x) + \phi(y). \]
 In other words, $\phi$ is a way of re-assigning names of the elements