Fix reference to deprecated SIGUP call (#1284)

This updates the instructions about sending a `SIGUP` call (deprecated) to `SIGUSR1`, and links to relevant docs: http://postgrest.org/en/v5.2/admin.html#schema-reloading

Per the docs:

> As of PostgREST v5.1 reloading with SIGHUP is deprecated, it’s still supported but will be removed in v6.0. SIGUSR1 should be used instead.

See [5.1.0 CHANGELOG](https://github.com/PostgREST/postgrest/blob/d32f373e1ee95ba68141a3f3a48ab8c94ea2453a/CHANGELOG.md#deprecated) or #724

.github/CONTRIBUTING.md

@@ -32,7 +32,7 @@ your contributions.
   then [find your logs](http://blog.endpoint.com/2014/11/dear-postgresql-where-are-my-logs.html).
 
 * If your database schema has changed while the PostgREST server is running,
-  send the server a `SIGHUP` signal or restart it to ensure the schema cache
+  [send the server a `SIGUSR1` signal](http://postgrest.org/en/v5.2/admin.html#schema-reloading) or restart it to ensure the schema cache
   is not stale. This sometimes fixes apparent bugs.
 
 ## Code