[DOC] Excluding From Coverage bad defaults from given example

[cboudereau]

Describe the bug
From the documentation:

The problem is that at least all async code is excluded from code coverage as it is stated in the dotnet documentation:

Expected behavior
The given example should not add unwanted behavior. Removing CompilerGeneratedAttribute from the example could be a good start

Actual behavior
By using the given example, the code coverage is higher than expected.