spec: Fix SnapshotSource commment

Comment in SnapshotSource make references as if it were a newly created
snapshot instead of a reference to a source snapshot.

Talking about "created snapshot" and how this id will be used in
subsequent calls may be confusing.

Comment is changed to reflect that it is an already existing source
snapshot.

Author: Akrog

csi.proto

@@ -214,11 +214,10 @@ message CreateVolumeRequest {
 // type fields MUST be specified.
 message VolumeContentSource {
   message SnapshotSource {
-    // Contains identity information for the created snapshot. This
-    // field is REQUIRED. The identity information will be used by
-    // the CO in subsequent calls to refer to the provisioned snapshot.
-    // Plugin is REQUIRED to support creating volume from snapshot if
-    // it supports the capability CREATE_DELETE_SNAPSHOT.
+    // Contains identity information for the existing source snapshot.
+    // This field is REQUIRED. Plugin is REQUIRED to support creating
+    // volume from snapshot if it supports the capability
+    // CREATE_DELETE_SNAPSHOT.
     string id = 1;
   }