docs(readme): fix NUT variables docs link

README.md

@@ -8,7 +8,7 @@ A sample [dashboard](dashboard/dashboard.json) for Grafana is also available
 ## Variables and information
 The variables exposed to a NUT client by the NUT system are the lifeblood of a deployment. These variables are consumed by this exporter and coaxed to Prometheus types.
 
- * See the [NUT documentation](https://networkupstools.org/docs/user-manual.chunked/apcs01.html) for a list of all possible variables
+ * See the [NUT documentation](https://networkupstools.org/docs/user-manual.chunked/_variables.html) for a list of all possible variables
  * Variables are set as prometheus metrics with the `ups` name added as a lable. Example: `ups.load` is set as `network_ups_tools_ups_load 100`
  * The exporter SHOULD be called with the ups to scrape set in the query string. Example: `https://127.0.0.1:9199/ups_metrics?ups=foo`
  * If the exporter scrapes NUT and detects more than one UPS, it is an error condition that will fail the scrape. In this case, use a variant of the scrape config example below for your environment