Update simplemde.js

Improve user experience when inserting image.

src/js/simplemde.js

@@ -1275,7 +1275,7 @@ var toolbarBuiltInButtons = {
 
 var insertTexts = {
 	link: ["[", "](#url#)"],
-	image: ["![](", "#url#)"],
+	image: ["![", "](#url#)"],
 	table: ["", "\n\n| Column 1 | Column 2 | Column 3 |\n| -------- | -------- | -------- |\n| Text     | Text     | Text     |\n\n"],
 	horizontalRule: ["", "\n\n-----\n\n"]
 };
@@ -2079,4 +2079,4 @@ SimpleMDE.prototype.toTextArea = function() {
 	}
 };
 
-module.exports = SimpleMDE;
+module.exports = SimpleMDE;