collector/indices.go: Fix golint issues.
collector/indices.go: Fix golint issues.

$ gometalinter --deadline 6m --vendor --disable-all --enable=golint collector/indices.go
collector/indices.go:38:6:warning: exported type Indices should have comment or be unexported (golint)
collector/indices.go:52:1:warning: exported function NewIndices should have comment or be unexported (golint)
collector/indices.go:903:1:warning: exported method Indices.Describe should have comment or be unexported (golint)
collector/indices.go:949:1:warning: exported method Indices.Collect should have comment or be unexported (golint)

justwatchcom/elasticsearch_exporter,ewr/elasticsearch_exporter