UsingCheckstyle.adoc: update documentation to include clicking `Apply`
UsingCheckstyle.adoc: update documentation to include clicking `Apply`

Clicking the Apply button after configuring the Checkstyle version is
not explicitly specified in documentation.

The Checkstyle-IDEA plugin will continue to use the previous version
if the button was not clicked to apply the changes.

Let's include clicking Apply button, after version configuration, in
the UsingCheckstyle.adoc's instructions.

se-edu/addressbook-level3