Fixed a unicode error.

I'm not sure how I got a funky unicode character in there, but it was
just an apostrophe in the comment. No big deal, but this fixes it so
that there is no warning message on Gitea.

src/ast.rs

@@ -45,7 +45,7 @@ pub struct OptimizedAst<T>
 {
    /// Node data in a linear layout (DFS or BFS order).
    pub nodes: Vec<NodeData<T>>,
-   /// Each node’s parent, if any.
+   /// Each node's parent, if any.
    pub parents: Vec<Option<NodeId>>,
    /// The traversal order the nodes are stored in.
    pub order: TraversalOrder