Remove unneeded A

Author: Veronica Ray

source/content/pages/10-css/1001-cascading-style-sheets.markdown

@@ -5,7 +5,7 @@ sort-order: 10
 
 
 # Cascading Style Sheets (CSS)
-A Cascading Style Sheet (CSS) files contain rules for how to display and 
+Cascading Style Sheet (CSS) files contain rules for how to display and 
 lay out the HTML content when it is rendered by a web browser.
 
 ## Why is CSS necessary?