Change the layout of the CCHttpClient article

1. Add cpp mark
2. Make subtitles easier to be seen
Fixed broken link to github repository.
Fix a typo in gui/overview/en.md
Merge pull request #78 from bunnyhero/readme-link-fix

Fixed broken link to github repository.
Merge pull request #77 from Pisces000221/patch-1

Change the layout of the CCHttpClient article