Fix error in Alerts component doc (twbs#23522)

Author: Jonathan Berger

docs/4.0/components/alerts.md

@@ -8,7 +8,7 @@ toc: true
 
 ## Examples
 
-Alerts are available for any length of text, as well as an optional dismiss button. For proper styling, use one of the four **required** contextual classes (e.g., `.alert-success`). For inline dismissal, use the [alerts jQuery plugin](#dismissing).
+Alerts are available for any length of text, as well as an optional dismiss button. For proper styling, use one of the eight **required** contextual classes (e.g., `.alert-success`). For inline dismissal, use the [alerts jQuery plugin](#dismissing).
 
 {% example html %}
 {% for color in site.data.theme-colors %}