Fix typo (#4756)

Author: frQQby

reference/image/functions/imagecolorset.xml

@@ -106,7 +106,7 @@ imagecolorallocate($im, 255, 0, 0);
 // Get the color index for the background
 $bg = imagecolorat($im, 0, 0);
 
-// Set the backgrund to be blue
+// Set the background to be blue
 imagecolorset($im, $bg, 0, 0, 255);
 
 // Output the image to the browser