change single quotes to double quotes

README.md

@@ -132,7 +132,7 @@ var simplemde = new SimpleMDE({
 		strikethrough: false,
 		underscoresBreakWords: true,
 	},
-    placeholder: 'Type here...',
+    placeholder: "Type here...",
 	previewRender: function(plainText) {
 		return customMarkdownParser(plainText); // Returns HTML from a custom parser
 	},

src/js/simplemde.js

@@ -1049,7 +1049,7 @@ SimpleMDE.prototype.render = function(el) {
 		extraKeys: keyMaps,
 		lineWrapping: (options.lineWrapping === false) ? false : true,
 		allowDropFileTypes: ["text/plain"],
-		placeholder: options.placeholder || el.getAttribute('placeholder') || ''
+		placeholder: options.placeholder || el.getAttribute("placeholder") || ""
 	});
 
 	if(options.toolbar !== false) {