fix example for mockImplementation description
fix example for mockImplementation description

Summary: Closes https://github.com/facebook/jest/pull/585

Reviewed By: svcscm

Differential Revision: D2592825

fb-gh-sync-id: 4c8679cbf8a76062870a080022c6d41017db5591

razh/jest,razh/jest,aaron-goshine/jest,alvinsj/jest,gagoman/jest,Daniel15/jest,cpojer/jest,ncuillery/jest,sampepose/jest,gagoman/jest,aaron-goshine/jest,aaron-goshine/jest,ncuillery/jest,skovhus/jest,blainekasten/jest,blainekasten/jest,facebook/jest,Daniel15/jest,yungsters/jest,ide/jest,cpojer/jest,sampepose/jest,ide/jest,mpontus/jest,skovhus/jest,yungsters/jest,bookman25/jest,blainekasten/jest,mpontus/jest,alvinsj/jest,facebook/jest,mpontus/jest,bookman25/jest,facebook/jest,Daniel15/jest,bookman25/jest,cpojer/jest,skawian/jest,ide/jest,skovhus/jest,skawian/jest