Replace deprecated uses of .pull-left & .pull-right in .media docs

We don't generally give examples of deprecated things.
Refs https://github.com/twbs/bootstrap/blob/1339ab7a130600330b1db4e3acae9760710a0a41/docs/_includes/components/media.html#L70
Refs https://github.com/twbs/bootlint/pull/153
CC: @mdo