Fix docstring for ModelVersionIdentifier (#216)

Signed-off-by: Mattt Zmuda <mattt@replicate.com>

Author: mattt

replicate/identifier.py

@@ -8,7 +8,7 @@
 
 class ModelVersionIdentifier(NamedTuple):
     """
-    A reference to a model version in the format owner/name:version.
+    A reference to a model version in the format owner/name or owner/name:version.
     """
 
     owner: str