Fix typo

Signed-off-by: laurentsimon <laurentsimon@google.com>

Author: laurentsimon

sigstore/_cli.py

@@ -686,7 +686,7 @@ def _sign(args: argparse.Namespace) -> None:
     with signing_ctx.signer(identity) as signer:
         for file, outputs in output_map.items():
             logger.debug(f"signing for {file.name}")
-            with file.open(mode="rb", buffering=0) as fio:
+            with file.open(mode="rb", buffering=0) as io:
                 try:
                     result = signer.sign(input_=io)
                 except ExpiredIdentity as exp_identity: