Added doctype declaration

In IE in particular, not including a doctype causes it to do all sorts of ridiculous things, like rendering in IE5 quirks mode. I am not sure why any developers would using IE, but my QA team found this and it's an easy enough fix and technically the right thing to do, so might as well fix it.
pirms 12 gadiem · abcd97d512
--- a/src/main/html/index.html
+++ b/src/main/html/index.html
@@ -1,3 +1,4 @@
 <!DOCTYPE html>
 <html>
 <head>
    <title>Swagger UI</title>