[CustomerAssignment] Fix typo when passing the company ID to the router

[aarongerig]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no

Before this PR the following URL is generated:

/admin/coreshop/customer-company-modifier/dispatch-existing-assignment/16685/undefined

Which leads to following error:

CoreShop\Bundle\CoreBundle\Controller\CustomerTransformerController::dispatchExistingAssignmentAction():
Argument #3 ($companyId) must be of type int, string given

With this PR, the correct company ID gets passed to the router and there generates the correct URL.

[dpfaffenbauer]

thanks @aarongerig