[css-values] Fix w3c#271 Error in bgpos example

Author: fantasai

css-values/Overview.bs

@@ -1661,7 +1661,7 @@ Computed Value</h4>
 
 		Due to this, 'background-position' preserves the percentage in a ''calc()''
 		rather than resolving it directly into a length,
-		so that an expression like ''background-position: left calc(50% + 20px)''
+		so that an expression like ''background-position: left calc(50% + 20px) center''
 		properly centers the background and then shifts it ''20px'' to the right,
 		rather than placing its <em>left edge</em> 20px off of center.
 	</div>
@@ -2261,6 +2261,7 @@ Changes</h2>
 		<li>Defined an empty <<url>> resolves to an invalid resource.
 		<li>Defined <a href="#calc-serialize">serialization of <css>calc()</css> values</a>.
 		<li>Defined <a>compatible units</a> and <a>canonical units</a> for serialization.
+		<li>Defined case-sensitivity of ''url()'' attribute argument to match attribute selectors.
 		<li>Added definition of <<ident>> notation to definition of <a>identifiers</a>.
 		<li>Added <<length-percentage>> as a shorthand for <<length>> | <<percentage>>,
 			along with equivalent productions for angles, numbers, times, and frequencies.