analyze-migration.py: fix a long standing typo

The parameters of '-d' can be either 'state' or 'desc', not 'dump'
as it is reported in the error message.

Fixes: b17425701d66 ("Add migration stream analyzation script")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211015131645.501281-2-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

scripts/analyze-migration.py

@@ -610,4 +610,4 @@ def default(self, o):
     dump.read(desc_only = True)
     print(jsonenc.encode(dump.vmsd_desc))
 else:
-    raise Exception("Please specify either -x, -d state or -d dump")
+    raise Exception("Please specify either -x, -d state or -d desc")