Fix IMAGEURL reference (#1025)

Co-authored-by: Seth G <[email protected]>

Author: github-actions[bot]

en/mapfile/template.txt

@@ -183,7 +183,7 @@ File Reference
 
 [img]
      Path (relative to document root) of the new image, just the image
-     name if IMAGE_URL is not set in the mapfile.
+     name if IMAGEURL is not set in the mapfile.
 
      In a map interface template, [img] is substituted with the path
      to the map image.  In a query results template, it is substituted