Fix package name check in publish_nuget.yml: GdipExtensions, not Extensions

The validation step never actually ran end-to-end before, so this typo
(checking for a nonexistent "OpenCvSharp5.Extensions" package instead of
"OpenCvSharp5.GdipExtensions") went unnoticed until the first real run.

Author: shimat

.github/workflows/publish_nuget.yml

@@ -101,7 +101,7 @@ jobs:
           }
 
           check_package "OpenCvSharp5.5.*.nupkg"
-          check_package "OpenCvSharp5.Extensions.*.nupkg"
+          check_package "OpenCvSharp5.GdipExtensions.*.nupkg"
           check_package "OpenCvSharp5.WpfExtensions.*.nupkg"
           check_package "OpenCvSharp5.runtime.win.[0-9]*.nupkg"
           check_package "OpenCvSharp5.runtime.win-arm64.[0-9]*.nupkg"