Link to REST API doc was missing extension .md

Change-Id: Idbb91ecb9a36b414893b65c71446cb8af6500bc8
Reviewed-on: https://review.gluster.org/20309
Tested-by: Yaniv Kaul <[email protected]>
Reviewed-by: Vijay Bellur <[email protected]>
Reviewed-by: Prashanth Pai <[email protected]>

Author: Yaniv Kaul

design/GlusterD2/GD2-Design.md

@@ -32,7 +32,7 @@ GD2 will also be a native ReST server, exposing cluster management interfaces vi
 
 The main management interface with GD2 will be a HTTP ReST interface. APIs will be provided for the management of peers, management of volumes, local-GlusterD management, monitoring (events) and long-running asynchronous operations.
 
-More details on the ReST-API can be found [here](ReST-API) (_note that this is still under active development_).
+More details on the ReST-API can be found [here](ReST-API.md) (_note that this is still under active development_).
 
 ### Gluster CLI