Make negated class explanation clearer

Some readers were—understandably—confused by the use of x, y, and z as
variables

chapters/character-classes.mdx

@@ -68,9 +68,7 @@ We can also "negate" these rules:
 
 The only difference between the first regex of this chapter and `/[^aeiou]/g` is the `^` immediately after the opening bracket. Its purpose is to negate the rules defined within the brackets. We are now saying:
 
-> "match any character that is _not_ any of `x`, `y` and `z`"
-
-Here, `x`, `y`, and `z` are either individual characters, ranges, or a combination of the two.
+> "match any character that is _not_ any of `a`, `e`, `i`, `o`, and `u`"
 
 ## Examples