docs: update null convention in ORM example, since @[nonull] is no longer needed (#20286)

doc/docs.md

@@ -4978,9 +4978,9 @@ import db.sqlite
 @[table: 'customers']
 struct Customer {
 	id        int    @[primary; sql: serial] // a field named `id` of integer type must be the first field
-	name      string @[nonull]
+	name      string
 	nr_orders int
-	country   string @[nonull]
+	country   string
 }
 
 db := sqlite.connect('customers.db')!
@@ -7153,4 +7153,4 @@ Assignment Operators
 +=   -=   *=   /=   %=
 &=   |=   ^=
 >>=  <<=  >>>=
-```
+```