[css-images-3] Fix typo (#3100)

Author: jiangying000

css-images-3/Overview.bs

@@ -284,7 +284,7 @@ Linear Gradients: the ''linear-gradient()'' notation {#linear-gradients}
 
 			This example illustrates visually how to calculate the <a>gradient line</a> from the rules above.
 			This shows the starting and ending point of the <a>gradient line</a>,
-			long with the actual gradient,
+			along with the actual gradient,
 			produced by an element with ''background: linear-gradient(45deg, white, black);''.
 
 			Notice how, though the starting point and ending point are outside of the box,