docs: update select-query-builder.md (typeorm#6681)

Found a typo on the "Adding WHERE expression" section: user.firstName is used in the SQL snippet where you actually meant user.id

docs/select-query-builder.md

@@ -342,7 +342,7 @@ createQueryBuilder("user")
 Which will produce the following SQL query:
 
 ```sql
-SELECT ... FROM users user WHERE user.firstName IN (1, 2, 3, 4)
+SELECT ... FROM users user WHERE user.id IN (1, 2, 3, 4)
 ```