Indices shards docs metrics name contains spelling mistake

[shaunmansell]

The metric tracking index shard doc totals is currently called elasticsearch_indices_shared_docs, where I would expect it's meant to be elasticsearch_indices_shards_docs.

Looks to be a result of:
https://github.com/justwatchcom/elasticsearch_exporter/blob/94988034cb1cb6b1ef7778e0586fe2d5aaea99e0/collector/indices.go#L952