[BUGFIX] Install tool documentation default installTollPassword

The fatal error message instructs to use array position EXT instead
of position BE for installToolPassword.
Replaces EXT with BE in the error message.

Fixes: #44682
Releases: 6.0, 6.1
Change-Id: Ic262e9c9838dc07dc523e9a85cf92e8f1f79e634
Reviewed-on: https://review.typo3.org/17960
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn