Fix use of output_path where input_path is the relevant reference. (#27025)

This seems like a copy and paste mistake from a few lines above.

Closes #27025

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/27025 from dsymonds:filepath-fix 4f7916edf0a594505adf1b2f94781a48f6e47928
PiperOrigin-RevId: 909995317

bazel/private/file_concat/main.cc

diff --git a/bazel/private/file_concat/main.cc b/bazel/private/file_concat/main.cc
index 32d49ba..3c7f32a 100644
--- a/bazel/private/file_concat/main.cc
+++ b/bazel/private/file_concat/main.cc
@@ -37,7 +37,7 @@
     std::string input_path(argv[i]);
     std::ifstream input(input_path, std::ifstream::binary);
     if (!input) {
-      std::cerr << "Could not open input file " << output_path << std::endl;
+      std::cerr << "Could not open input file " << input_path << std::endl;
       return kIOError;
     }