uhttpd: rename certificate defaults section

Now that the uhttpd init script can generate certificates using openssl as
well, update the section name and related comment to be more generic.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/network/services/uhttpd/files/uhttpd.config

@@ -104,8 +104,8 @@ config uhttpd main
 #	option no_ubusauth	0
 
 
-# Certificate defaults for px5g key generator
-config cert px5g
+# Defaults for automatic certificate and key generation
+config cert defaults
 
 	# Validity time
 	option days		730