Fix right margin of the carousel "next" button

The calculated offset of carousel's right-arrow differs with left-arrow on large viewport sizes. The problem was well described and illustrated [here](https://github.com/twbs/bootstrap/pull/13242#issuecomment-39086675) and [here](https://github.com/twbs/bootstrap/pull/13242#issuecomment-39087556) by @hnrch02. This PR is just a missing part of #13242