docs: add missing uint64 count prefix to ServersResponse docstring

The docstring said "Body: repeated (...)" but the actual wire format
starts with a uint64 node count, matching FilesResponse's documented
format.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/messages/responses.py

@@ -497,7 +497,7 @@ class NodeInfo:
 class ServersResponse(Message):
     """Cluster servers response.
 
-    Body: repeated (uint64 node_id, text address, uint64 role)
+    Body: uint64 count, then repeated (uint64 node_id, text address, uint64 role)
     """
 
     MSG_TYPE: ClassVar[int] = ResponseType.SERVERS