Update default message for "no-focusable-content" check

From https://github.com/dequelabs/axe-core/pull/3194#discussion_r737402759

The ```default``` message in the [no-focusable-content check ](https://github.com/dequelabs/axe-core/blob/b985776b6fdb2c96f40df38cf86f7241039d4f5b/lib/checks/keyboard/no-focusable-content.json)mentions the cause of the failure where as the ```notHidden``` message mentions the effect of the failure

Product: axe-core

Expectation: Modify message to be consistent with other messages

Actual: 'Element has focusable descendants'

Motivation: Having consistent messages

axe-core version: develop