docs(tooltip): change bottom to below (#2104)

Author: andrewseguin

src/lib/tooltip/README.md

@@ -6,7 +6,7 @@ The positions `before` and `after` should be used instead of `left` and `right`
 ### Examples
 A button with a tooltip
 ```html
-<button md-tooltip="some message" tooltip-position="bottom">Button</button>
+<button md-tooltip="some message" tooltip-position="below">Button</button>
 ```