Update navbar.less

In the first place i wonder why the docs suggest to use a `p` (block) instead of a `span` (inline) tag.
Using `p` tags seems a reason to set margins instead of padding (cause the `p` tag has a `bottom-margin` by default).
Also calling the `.navbar-vertical-align()` mixin does not make much sense, cause .navbar-text should only contain text and its height is keeps fixed by `@line-height-computed always`.