Improve comment in gram.y.

"As so-and-so" isn't bad English, but it has a faintly archaic
whiff to it, and confuses some non-native speakers.  Write
"Like so-and-so" instead.

Per complaint from Tatsuo Ishii.

Discussion: https://postgr.es/m/20240623.130154.1867056921698616251.t-ishii@sranhm.sra.co.jp.sranhm

src/backend/parser/gram.y

@@ -15667,7 +15667,7 @@ func_expr: func_application within_group_clause filter_clause over_clause
 		;
 
 /*
- * As func_expr but does not accept WINDOW functions directly
+ * Like func_expr but does not accept WINDOW functions directly
  * (but they can still be contained in arguments for functions etc).
  * Use this when window expressions are not allowed, where needed to
  * disambiguate the grammar (e.g. in CREATE INDEX).