ox-md: Fix missing ':' in exported markdown image links

* lisp/ox-md.el (org-md-link): Append ':' to the type of non-file
  image links.

lisp/ox-md.el

@@ -395,7 +395,7 @@ a communication channel."
 		       (org-export-get-reference destination info))))))))
      ((org-export-inline-image-p link org-html-inline-image-rules)
       (let ((path (let ((raw-path (org-element-property :path link)))
-		    (cond ((not (equal "file" type)) (concat type raw-path))
+		    (cond ((not (equal "file" type)) (concat type ":" raw-path))
 			  ((not (file-name-absolute-p raw-path)) raw-path)
 			  (t (expand-file-name raw-path)))))
 	    (caption (org-export-data