correction to B2 config in the example

This example was missing the `env:` block in YAML, which if missing, caused `autorestic check` to compain:

``` * 'Backends[remote]' has invalid keys: b2_account_id, b2_account_key```

docs/markdown/config.md

@@ -31,6 +31,7 @@ backends:
   remote:
     type: b2
     path: 'myBucket:backup/home'
+    env:
       B2_ACCOUNT_ID: account_id
       B2_ACCOUNT_KEY: account_key