[css-pseudo-4] Use slightly more self-consistent informal syntax in the note. w3c#5830

Author: fantasai

css-pseudo-4/Overview.bs

@@ -333,7 +333,7 @@ First-Letter Text: ''::first-letter'' pseudo-element and its ''::prefix'' and ''
   <div class="note">
     Informally represented, the first-letter text’s pattern here is
     <code highlight="bnf" style="font-family: inherit">
-    {S,P}* {L,N}+ {S,P&minus;(Ps+Pd)}*</code>
+    (S|P)* (L|N)+ (S|P^(Ps|Pd))*</code>
     or, alternatively,
     <code highlight="bnf" style="font-family: inherit">
     [S P]* [L N]+ [[S] [P--[Ps Pd]]]*</code>