apacheGH-39598: [C#] Fix verification script (apache#39605)

### What changes are included in this PR?

The verification script is modified to look for the versions of .NET now supported by the package.

### Are these changes tested?

Manually tested the verification command.

* Closes: apache#39598

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>

dev/release/verify-release-candidate.sh

@@ -878,10 +878,10 @@ test_csharp() {
   fi
 
   if [ "${SOURCE_KIND}" = "local" ]; then
-    echo "Skipping sourelink verification on local build"
+    echo "Skipping sourcelink verification on local build"
   else
-    dotnet tool run sourcelink test artifacts/Apache.Arrow/Release/netstandard1.3/Apache.Arrow.pdb
-    dotnet tool run sourcelink test artifacts/Apache.Arrow/Release/netcoreapp3.1/Apache.Arrow.pdb
+    dotnet tool run sourcelink test artifacts/Apache.Arrow/Release/netstandard2.0/Apache.Arrow.pdb
+    dotnet tool run sourcelink test artifacts/Apache.Arrow/Release/net6.0/Apache.Arrow.pdb
   fi
 
   popd