fix offsetTop is negative

Author: jljsj33

examples/link.js

@@ -43,12 +43,12 @@ class Demo extends React.Component {
           <p>Ant Motion</p>
         </div>
         <div className="nav-wap">
-          <Link className="nav-list" location="page0" offsetTop={-58}
+          <Link className="nav-list" location="page0"
             onFocus={this.onFocus.bind(this)}
           >
             Example
           </Link>
-          <Link className="nav-list" location="page1" offsetTop={-58}
+          <Link className="nav-list" location="page1" offsetTop={158}
             onFocus={this.onFocus.bind(this)}
           >
             Example2

src/ScrollLink.jsx

@@ -45,7 +45,7 @@ class ScrollLink extends React.Component {
     const elementDom = mapped.get(this.props.location);
     const elementRect = elementDom.getBoundingClientRect();
     this.scrollTop = currentScrollTop();
-    const toTop = Math.round(elementRect.top) - Math.round(docRect.top) + this.props.offsetTop;
+    const toTop = Math.round(elementRect.top) - Math.round(docRect.top) - this.props.offsetTop;
     this.toTop = this.props.toShowHeight ?
     toTop - transformArguments(this.props.showHeightActive)[0] : toTop;
     this.initTime = Date.now();