[BUGFIX] Fix typo in ChecboxElement

When calling the deprected method getSingleField_typeCheck
a wrong class name is called.
It has to be CheckboxElement instead of ChecboxElement

Resolves: #62149
Releases: master
Change-Id: Ie3db582ea40bb5e55ac4f59ac6c89364304f5723
Reviewed-on: http://review.typo3.org/33249
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>