Fix broken echo filter link in README (envoyproxy#58)

Signed-off-by: Venil Noronha <[email protected]>

README.md

@@ -2,7 +2,7 @@
 
 This project demonstrates the linking of additional filters with the Envoy binary.
 A new filter `echo2` is introduced, identical modulo renaming to the existing
-[`echo`](https://github.com/envoyproxy/envoy/blob/master/source/common/filter/echo.h)
+[`echo`](https://github.com/envoyproxy/envoy/blob/master/source/extensions/filters/network/echo/echo.h)
 filter. Integration tests demonstrating the filter's end-to-end behavior are
 also provided.