Fix clippy warning: collapse nested if in Day 4

src/bin/04.rs

@@ -41,10 +41,10 @@ fn count_neighbors(grid: &[Vec<Cell>], row: usize, col: usize) -> usize {
             }
             let nr = row as i32 + dr;
             let nc = col as i32 + dc;
-            if nr >= 0 && nr < rows && nc >= 0 && nc < cols {
-                if grid[nr as usize][nc as usize].is_paper() {
-                    count += 1;
-                }
+            if nr >= 0 && nr < rows && nc >= 0 && nc < cols
+                && grid[nr as usize][nc as usize].is_paper()
+            {
+                count += 1;
             }
         }
     }