[BUG][aspnetcore] Documentation : operationModifier can't be partial

[tmakin]

Description

Very minor issue.

The help text for aspnetcore generator suggests that the operationModifier option can be partial. This is not possible in c# and is not implemented in the generator (it's just the help text that is wrong)
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AspNetCoreServerCodegen.java#L73

openapi-generator version

4.2.2

Steps to reproduce

Using CLI:

> openapi-generator config-help -g aspnetcore

...
 operationModifier
            Operation Modifier can be virtual, abstract or partial (Default: virtual)
...

Suggest a fix

Update help text as follows:
private CliOption operationModifier = new CliOption(OPERATION_MODIFIER, "Operation Modifier can be virtual or abstract");

[auto-labeler]

👍 Thanks for opening this issue!
🏷 I have applied any labels matching special text in your issue.

The team will review the labels and make any necessary changes.

[wing328]

Thanks for the fix by @Sshnyari

@tmakin please pull the latest master or snapshot version to give it a try.