Docs: Provider docs wrong variable name

[cdsre]

The provider now supports setting credentials via environment variables. This was introduced in #239. However the provider docs state to set the env var as CAPELLA_AUTH_TOKEN. However in the code its actually looking for CAPELLA_AUTHENTICATION_TOKEN

	if config.AuthenticationToken.IsNull() {
		config.AuthenticationToken = types.StringValue(os.Getenv("CAPELLA_AUTHENTICATION_TOKEN"))
	}

The documentation should be updated to reflect the code.