[FBcode->GH] Use proper variable for tls key file for avformat options (#3374)

Summary:
Noticed this copy-paste error while reading D26026259 (https://github.com/pytorch/vision/commit/f16322b596c7dc9e9d67d3b40907694f29e16357) for inspiration to solve a similar problem

Created from Diffusion's 'Open in Editor' feature.

Reviewed By: datumbox

Differential Revision: D26373056

fbshipit-source-id: d67f46243a280e8a59d12bad9dd730c1a3d512c9

torchvision/csrc/io/decoder/decoder.cpp

@@ -320,7 +320,7 @@ bool Decoder::init(
       av_dict_set(&options, "cert_file", params_.tlsCertFile.data(), 0);
     }
     if (!params_.tlsKeyFile.empty()) {
-      av_dict_set(&options, "key_file", params_.tlsCertFile.data(), 0);
+      av_dict_set(&options, "key_file", params_.tlsKeyFile.data(), 0);
     }
   }