Fix developer docs link (#18)

Author: mauerbac

README.md

@@ -40,7 +40,7 @@ The supplied `pom` files on Bintray define module dependencies.
 
 ### Using the SDK
 
-See the Optimizely server-side testing [developer documentation](http://developers.optimizely.com/server/reference/index) to learn how to set
+See the Optimizely server-side testing [developer documentation](http://developers.optimizely.com/server/reference/index.html) to learn how to set
 up your first custom project and use the SDK. **Please note that you must be a member of the private server-side testing beta to create custom
 projects and use this SDK.**