Add parenthesis around left hand side of companion

Parenthesis are not needed around this expression but my editor is going
mad and cannot format the code without them. Since it does not hurt
readability add parenthesis around the expression.

src/util/taproot.rs

@@ -510,7 +510,7 @@ impl TaprootBuilder {
         // We cannot insert a leaf at a lower depth while a deeper branch is unfinished. Doing
         // so would mean the add_leaf/add_hidden invocations do not correspond to a DFS traversal of a
         // binary tree.
-        if depth as usize + 1 < self.branch.len() {
+        if (depth as usize + 1) < self.branch.len() {
             return Err(TaprootBuilderError::NodeNotInDfsOrder);
         }