Update solutions/usaco-646.mdx

Co-authored-by: ChuangSheep <44814054+ChuangSheep@users.noreply.github.com>

Author: ryanchou-dev

solutions/usaco-646.mdx

@@ -35,7 +35,7 @@ using namespace std;
 //BeginCodeSnip{DSU}
 struct DSU {
 	vector<int> e;
-	DSU(int N) { e = vector<int>(N, -1); }
+	DSU(int N): e(N, -1) { }
 
 	// get representive component (uses path compression)
 	int get(int x) { return e[x] < 0 ? x : e[x] = get(e[x]); }