fix attribute name to fit-to-markers

The `fitToMarkers` property is in the plural.
Fix #374

Author: nidelson

README.md

@@ -24,7 +24,7 @@ If you are seeking a smaller, simplified version of `google-map`, we recommend u
 ```
 -->
 ```html
-<google-map fit-to-marker api-key="AIzaSyD3E1D9b-Z7ekrT3tbhl_dy8DCXuIuDDRc">
+<google-map fit-to-markers api-key="AIzaSyD3E1D9b-Z7ekrT3tbhl_dy8DCXuIuDDRc">
   <google-map-marker latitude="37.78" longitude="-122.4" draggable="true"></google-map-marker>
 </google-map>
 ```