Comment out toot_visibility in examples

(to stress that it's optional)

README.md

@@ -40,8 +40,8 @@ Alternatively you can donate cryptocurrencies:
         instance_url=https://mastodon.social
         user_credentials=feed2toot_usercred.txt
         client_credentials=feed2toot_clientcred.txt
-        ; Default visibility is public, but you can override it
-        toot_visibility=unlisted
+        ; Default visibility is public, but you can override it:
+        ; toot_visibility=unlisted
 
         [cache]
         cachefile=cache.db