fixed variable names in SearchApiSolrBackend::extractResults()

[mkalkbrenner]

The variable names in SearchApiSolrBackend::extractResults() are really confusing:

• $resultset contains an instance of \Solarium\QueryType\Select\Result\Result
• $results contains an instance implementing \Drupal\search_api\Query\ResultSetInterface

I suggest to rename the variables to better reflect their nature:

• $resultset becomes $result
• $results becomes $result_set

[amateescu]

Yeah, those variable names confused me as well when I ported the module. They look better now :)