Fix old values in operations.md

`from: 0` throws
```
level=error ts=2019-08-07T12:02:15.385981107Z caller=main.go:48 msg="error loading config" filename=/etc/loki/local-config.yaml err="parsing time \"0\" as \"2006-01-02\": cannot parse \"0\" as \"2006\""
```

`store: dynamo` throws:
```
level=error ts=2019-08-07T11:59:01.688427565Z caller=main.go:71 msg="error initialising loki" err="error initialising module: table-manager: Unrecognized storage client dynamo, choose one of: aws, cassandra, inmemory, gcp, bigtable, bigtable-hashed"
```

docs/operations.md

@@ -137,8 +137,8 @@ Example config for using S3 & DynamoDB:
 ```yaml
 schema_config:
   configs:
-    - from: 0
-      store: dynamo
+    - from: 2018-04-15
+      store: aws
       object_store: s3
       schema: v9
       index: