Fix wrong value type in example config

HSTS maxAge has to be an integer, not a string.

Fixes https://github.com/hackmdio/codimd/issues/1159

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

config.json.example

@@ -20,7 +20,7 @@
         "loglevel": "info",
         "hsts": {
             "enable": true,
-            "maxAgeSeconds": "31536000",
+            "maxAgeSeconds": 31536000,
             "includeSubdomains": true,
             "preload": true
         },