[Docs] Remove extraneous text (elastic#40914)

Removes text that was likely introduced by copy/paste error.

Author: dedemorton

docs/reference/upgrade.asciidoc

@@ -37,7 +37,6 @@ deprecation warnings are logged when the log level is set to `WARN`.
 to your code and configuration for {version}.
 . If you use custom plugins, make sure compatible versions are available.
 . Test upgrades in a dev environment before upgrading your production cluster.
-before upgrading.
 . <<modules-snapshots,Back up your data!>> You must have a snapshot of your
 data to roll back to an earlier version.