Correct a comment in AuthExplicitComputeEngine().
Correct a comment in AuthExplicitComputeEngine().

Fixes https://github.com/GoogleCloudPlatform/dotnet-docs-samples/issues/337
jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples