Use ?NNN format instead of ?

https://sqlite.org/lang_expr.html#parameters
> But because it is easy to miscount the question marks, the use of this parameter format is discouraged. Programmers are encouraged to use one of the symbolic formats below or the ?NNN format above instead.

src/transaction.rs

@@ -718,7 +718,7 @@ mod test {
     }
 
     fn insert(x: i32, conn: &Connection) -> Result<usize> {
-        conn.execute("INSERT INTO foo VALUES(?)", [x])
+        conn.execute("INSERT INTO foo VALUES(?1)", [x])
     }
 
     fn assert_current_sum(x: i32, conn: &Connection) -> Result<()> {