Correct alignment of meta block in template

That way, it is rendered as <pre> by markdown.

src/mainwindow.c

@@ -2,10 +2,10 @@
 #include "mdiary.h"
 
 const gchar *default_template[] = {"# Title\n"
-				   "Date: ", "\0",
+				   "       Date: ", "\0",
 				   "\n"
-				   "Summary: \n"
+				   "    Summary: \n"
-				   "Tags: \n"
+				   "       Tags: \n"
 				   "\n"
 				   "Text",
 				   NULL};