Update zend.view.helpers.partial.rst

PartialLoop doesn't assign a variable called partialCounter to your view-script (as is also used within the test scripts). The actual way to get the currency position is through: $this->partialLoop()->getPartialCounter() as I've altered into the documentation.
Merge branch 'hotfix/971'

Close #971
Merge pull request #973 from intellix/patch-2

Update zend.view.helpers.partial.rst
Merge branch 'hotfix/973' into develop

Forward port #973