Storage: Create a code block for example command.
The current doc is broken:
@@ -168,6 +168,7 @@ When a new recording rule is created, there is no historical data for it. Record
To see all options, use: `$ promtool tsdb create-blocks-from rules --help`.
Example usage:
+
```
$ promtool tsdb create-blocks-from rules \
--start 1617079873 \
