fix: return type $240 (#400)

Author: ruyadebaozi

problems/240.search-a-2-d-matrix-ii.md

@@ -69,7 +69,7 @@ JavaScript Code:
  * @return {boolean}
  */
 var searchMatrix = function(matrix, target) {
-    if (!matrix || matrix.length === 0) return 0;
+    if (!matrix || matrix.length === 0) return false;
 
     let colIndex = 0;
     let rowIndex = matrix.length - 1;