Update font to Roboto

Replace the previously used font in the style.css file with
Roboto. The original font was not widely supported across
different platforms and browsers, which could lead to
inconsistent user experiences. By switching to Roboto, we
ensure better compatibility and a more uniform appearance
for users accessing the application.

flaskr/static/style.css

@@ -13,7 +13,7 @@ body {
 }
 
 h1, h2, h3, h4, h5, h6 {
-  font-family: 'Roboto Slab', serif;
+  font-family: 'Roboto', serif;
   color: #377ba8;
   margin: 1rem 0;
 }