[css-fonts-4][editorial] remove duplicated text

Author: svgeesus

css-fonts-4/Overview.bs

@@ -561,8 +561,7 @@ Generic font families</h4>
 
 	        ''monospace'' must always map to at least one matched font face.
 
-	        Note: ''monospace'' must always map to at least one matched font face.
-	        However, no guarantee is placed on the character coverage of that font
+	        Note: No guarantee is placed on the character coverage of that font
 	        face. Therefore, the font ''monospace'' is mapped to may not end up being
 	        used for all content.