Fix invalid archive error message

Backporting b6a7a0806e8a563961e929a24d71b6be55f16c70 to 1.1-maint

src/borg/helpers.py

@@ -1205,7 +1205,7 @@ def archivename_validator():
     def validator(text):
         text = replace_placeholders(text)
         if '/' in text or '::' in text or not text:
-            raise argparse.ArgumentTypeError('Invalid repository name: "%s"' % text)
+            raise argparse.ArgumentTypeError('Invalid archive name: "%s"' % text)
         return text
     return validator