fix(docs): close code block in query_builder.rs (launchbadge#2067)

@danielakhterov and I were playing around with counting lines using regex and noticed that SQLx had an odd number of ` ``` ` and got a little nerd-sniped trying to find it.

Author: abonander

sqlx-core/src/query_builder.rs

@@ -379,7 +379,8 @@ where
  /// // 65535 / 4 = 16383 (rounded down)
  /// // 16383 * 4 = 65532
  /// assert_eq!(arguments.len(), 65532);
- /// }
+ /// }
+ /// ```
  pub fn push_tuples<I, F>(&mut self, tuples: I, mut push_tuple: F) -> &mut Self
  where
  I: IntoIterator,