Use string operations, not idx(), for string indexing.
Use string operations, not idx(), for string indexing.

Summary: Missed this in review (D1715), idx() does not operate on strings (maybe
it does in HPHP/i?).

Test Plan: Faked an editable lint warning, ran "arc lint".

Reviewers: Koolvin, andrewjcg, btrahan

Reviewed By: Koolvin

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1728

phacility/arcanist,folsom-labs/arcanist,jra3/arcanist,Khan/arcanist,wxstars/arcanist,sneakpeeq/arcanist,phacility/arcanist,memsql/arcanist,alexmv/arcanist,wikimedia/phabricator-arcanist,alexmv/arcanist,kevinseelbach/arcanist,jra3/arcanist,eSpark/arcanist,PageUpPeopleOrg/arcanist-patches,calblueprint/arcanist,julkiewicz/arcanist,PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,Symplicity/arcanist,alexmv/arcanist,dguo-coursera/arcanist,wxstars/arcanist,folsom-labs/arcanist,hach-que/arcanist,calblueprint/arcanist,julkiewicz/arcanist,Khan/arcanist,codelabs-ch/arcanist,calblueprint/arcanist,wikimedia/phabricator-arcanist,uber/arcanist,wharris/arcanist,aik099/arcanist,wxstars/arcanist,kevinseelbach/arcanist,fivestars/arcanist,jra3/arcanist,memsql/arcanist,Khan/arcanist,KieranHayes/arcanist,codelabs-ch/arcanist,r4nt/arcanist,codelabs-ch/arcanist,uber/arcanist,chundongwang/arcanist,eSpark/arcanist,coursera/arcanist,hach-que/arcanist,codelabs-ch/arcanist,hach-que/arcanist,fivestars/arcanist,zhihu/arcanist,wxstars/arcanist,haskell-infra/arcanist,sneakpeeq/arcanist,chundongwang/arcanist,wikimedia/phabricator-arcanist,r4nt/arcanist,haskell-infra/arcanist,phacility/arcanist,julkiewicz/arcanist,wikimedia/phabricator-arcanist,zhihu/arcanist,dguo-coursera/arcanist,sneakpeeq/arcanist,phacility/arcanist,jra3/arcanist,folsom-labs/arcanist,PageUpPeopleOrg/arcanist-patches,eSpark/arcanist,coursera/arcanist,KieranHayes/arcanist,coursera/arcanist,haskell-infra/arcanist,r4nt/arcanist,memsql/arcanist,wharris/arcanist,aik099/arcanist,uber/arcanist,fivestars/arcanist,wharris/arcanist,dguo-coursera/arcanist,PageUpPeopleOrg/arcanist-patches,haskell-infra/arcanist,hach-que/arcanist,calblueprint/arcanist,coursera/arcanist,codelabs-ch/arcanist,zhihu/arcanist,aik099/arcanist,sneakpeeq/arcanist,julkiewicz/arcanist,Symplicity/arcanist,aik099/arcanist,kevinseelbach/arcanist,kevinseelbach/arcanist,Symplicity/arcanist,KieranHayes/arcanist,folsom-labs/arcanist,Khan/arcanist,chundongwang/arcanist,eSpark/arcanist,zhihu/arcanist,alexmv/arcanist,chundongwang/arcanist,fivestars/arcanist,KieranHayes/arcanist,r4nt/arcanist,uber/arcanist,wharris/arcanist,Symplicity/arcanist,phacility/arcanist,memsql/arcanist