[css-align-3] Replace ASCII with SVG.

Author: tabatkins

css-align-3/Overview.bs

@@ -986,17 +986,10 @@ The 'justify-content' and 'align-content' Properties</h3>
 	by changing the <a>static-position containing block</a>’s
 	<a>content-distribution properties</a>.
 
-    <pre>
-	###############################################
-	# abspos containing block                     #
-	#   +----------------------------------+      #
-	#   | staticpos containing block       |      #
-	#   |                                  |      #
-	#   |                                  |      #
-	#<--|start-align available space -->   |      #
-	#   |<----- end-align available space -|-->   #
-	#   |<- center-align available space ->|      #
-	</pre>
+	<figure>
+		<img src="images/place-content-abspos.svg" width=480 height=330 alt="">
+		<figcaption>An illustration of the above rules.</figcaption>
+	</figure>
 </div>
 
 ### Multicol Containers ### {#distribution-multicol}