[Azure Core] Readme content and sample code of Azure Core is incorrect

[wantedfast]

Section Link

Comments: Variable name not suitable, change it to allSecrets might be good for here.

Section Link
https://github.com/Azure/azure-sdk-for-net/tree/master/sdk/core/Azure.Core#consuming-long-running-operations-using-operationt

Comments: Should use async method here.

await client.StartDeleteSecretAsync("SecretName");

Section Link

Comments: Link not found, correct it to this.

Section Link

Comments: Should use async method here.

await client.StartDeleteSecretAsync("SecretName");

5,
Section Link

Comments: Link not found, correct it to this ?

[jongio]

1. allSecretProperties

Please PR.