Fix links in readme

Author: zeusdeux

Readme.md

@@ -13,9 +13,9 @@ viewport.
 - [API docs](#api)
   - [Return value](#return-value)
   - [Input options](#options)
-    - [options.threshold](#options.threshold)
-    - [options.target](#options.target)
-    - [options.viewport](#options.viewport)
+    - [options.threshold](#optionsthreshold)
+    - [options.target](#optionstarget)
+    - [options.viewport](#optionsviewport)
     - [options.{modTop, modRight, modBottom, modLeft}](#optionsmodtop-modright-modbottom-modleft)
 - [Example usage](#example-usage)
 - [Tasks](#tasks)