Use --decode for base64 command

`-d` is used for the Linux version of the command but in the Mac/BSD version they use `-D`.

Using `--decode` we are sure that the flag is compatible with both.

docs/user-guide/secrets/index.md

@@ -158,7 +158,7 @@ type: Opaque
 Decode the password field:
 
 ```shell
-$ echo "MWYyZDFlMmU2N2Rm" | base64 -d
+$ echo "MWYyZDFlMmU2N2Rm" | base64 --decode
 1f2d1e2e67df
 ```