docs: add promtool version in rule migration cmd (prometheus#6989)

`promtool update rules` is no longer available after Prometheus 2.5.
Document this to avoid confusion such as in prometheus#6988.

Signed-off-by: Christian Hoffmann <mail@hoffmann-christian.info>

docs/migration.md

@@ -120,7 +120,7 @@ new format. For example:
 $ promtool update rules example.rules
 ```
 
-Note that you will need to use promtool from 2.0, not 1.8.
+You will need to use `promtool` from [Prometheus 2.5](https://github.com/prometheus/prometheus/releases/tag/v2.5.0) as later versions no longer contain the above subcommand.
 
 ## Storage