fix(fmt): Indent trailing brace of nested nodes. (#49)

In 4.3.0, the Display output for nested nodes always puts the trailing
braces on the left margin:

    a {
        b {
            c {
    }
    }
    }

This commit indents the trailing brace to match the node name, at least
when explicit trailing text is not provided.

src/node.rs

@@ -513,7 +513,7 @@ impl KdlNode {
                 writeln!(f)?;
             }
             children.stringify(f, indent + 4)?;
-            write!(f, "}}")?;
+            write!(f, "{:indent$}}}", "", indent = indent)?;
         }
         if let Some(trailing) = &self.trailing {
             write!(f, "{}", trailing)?;