typo: remove unnecessary blank line in spec.md

Author: HeGaoYuan

spec.md

@@ -399,11 +399,9 @@ service Node {
   rpc NodeGetVolumeStats (NodeGetVolumeStatsRequest)
     returns (NodeGetVolumeStatsResponse) {}
 
-
   rpc NodeExpandVolume(NodeExpandVolumeRequest)
     returns (NodeExpandVolumeResponse) {}
 
-
   rpc NodeGetCapabilities (NodeGetCapabilitiesRequest)
     returns (NodeGetCapabilitiesResponse) {}