Updated Imagine thumb plugin description

Q | A
------------- | ---
Bug fix? | no
New feature? | no
BC breaks? | no
Deprecations? | no
Tests pass? | -
Fixed tickets | -
Refs tickets | -
License | MIT
Doc PR | -

In https://github.com/zikula/core/blob/1.3/src/plugins/Imagine/lib/Imagine/Manager.php#L416

the thumbnail modes that can be chosen are inset or outbound, not outset. It is also described quite poorly in the Imagine docs, see also https://github.com/avalanche123/Imagine/issues/91

The working can be seen on slide 28-30 of https://speakerdeck.com/avalanche123/introduction-to-imagine