merge: preserve ascii-art formatting in Merge::flatten doc-comment

Rustdoc was losing the formatting of these merge diagrams; preserve it
with code-block formatting.

Author: steadmon

lib/src/merge.rs

@@ -486,15 +486,19 @@ impl<T> Merge<Merge<T>> {
     ///
     /// Let's say we have a 3-way merge of 3-way merges like this:
     ///
+    /// ```
     /// 4 5   7 8
     ///  3     6
     ///    1 2
     ///     0
+    /// ```
     ///
     /// Flattening that results in this 9-way merge:
     ///
+    /// ```
     /// 4 5 0 7 8
     ///  3 2 1 6
+    /// ```
     pub fn flatten(self) -> Merge<T> {
         let mut outer_values = self.values.into_iter();
         let mut result = outer_values.next().unwrap();