[cpp.embed.gen] Fix typo in example

Author: jensmaurer

source/preprocessor.tex

@@ -975,7 +975,7 @@
   struct { double e, f, g; } x;
   double h, i, j;
 };
-is x = {
+s x = {
 // well-formed if the directive produces nine or fewer values
 #embed "s.dat"
 };