fixing the MIME type error in loading override CSS file in storybook

Author: NightWing1998

.storybook/preview-head.html

@@ -1,5 +1,6 @@
 <link rel="stylesheet"
-	href="assets/styles/salesforce-lightning-design-system.css" />
+	href="assets/styles/salesforce-lightning-design-system.css"
+	type="text/css" />
 <link rel="stylesheet" href="assets/styles/override.css" type="text/css" />
 <style>
 	html {