Fix typos, grammar and flow in 04.2.md
Fix typos, grammar and flow in 04.2.md

faxriddin/build-web-application-with-golang,astaxie/build-web-application-with-golang,songqii/build-web-application-with-golang,admpub/build-web-application-with-golang,songqii/build-web-application-with-golang,admpub/build-web-application-with-golang,WangKaimin/build-web-application-with-golang,WangKaimin/build-web-application-with-golang,faxriddin/build-web-application-with-golang,admpub/build-web-application-with-golang,astaxie/build-web-application-with-golang,astaxie/build-web-application-with-golang,admpub/build-web-application-with-golang