fix(CrosshairsTool.js): Fixed mousedown callback name (#1376)

Crosshairs would not work on click or mousedown because the callback name had been misspelled.

fix #1375

src/tools/CrosshairsTool.js

@@ -31,7 +31,7 @@ export default class CrosshairsTool extends BaseTool {
 
     super(props, defaultProps);
 
-    this.mouseDownCallback = this._chooseLocation.bind(this);
+    this.preMouseDownCallback = this._chooseLocation.bind(this);
     this.mouseDragCallback = this._chooseLocation.bind(this);
     this.touchDragCallback = this._chooseLocation.bind(this);
   }