[css-backgrounds-3] Fix example 15

"background: ... / 10em ... " is equivalent to "background-size: 10em auto;", not "background-size: 10em 10em;".

Author: CoderMJLee

css-backgrounds-3/Overview.bs

@@ -1232,7 +1232,7 @@ body {
 p {
     background-color: gray;
     background-position: 40% 50%;
-    background-size: 10em 10em;
+    background-size: 10em auto;
     background-repeat: round;
     background-clip: border-box;
     background-origin: border-box;

css-backgrounds-3/Overview.html

@@ -1310,7 +1310,7 @@ <h3 class="heading settled" data-level="3.10" id="the-background"><span class="s
 <pre>p {
     background-color: gray;
     background-position: 40% 50%;
-    background-size: 10em 10em;
+    background-size: 10em auto;
     background-repeat: round;
     background-clip: border-box;
     background-origin: border-box;