Update README.md

WARNING! The "vault auth ARG" command is deprecated and is now a subcommand
for interacting with auth methods. To authenticate locally to Vault, use
"vault login" instead. This backwards compatibility will be removed in Vault
0.11 (or later).

README.md

@@ -111,7 +111,7 @@ the Vault is now unsealed:
 We can use the `Initial Root Token` from above to auth with the Vault:
 
 ```bash
-$ vault auth
+$ vault login
 Token (will be hidden):
 Successfully authenticated! You are now logged in.
 token: 5a4a7e11-1e2f-6f76-170e-b8ec58cd2da5