Add missing metrics_path to multi-target example

Addresses issue 815:
#815

Like the author of the issue, I also used the example in the README file
to setup scraping with Prometheus only to discover that by default the
/metrics end-point is scraped instead.

Signed-off-by: Thomas Venieris <thomas.venieris@gmail.com>

README.md

@@ -56,6 +56,7 @@ Sample config file for multiple configurations
 On the prometheus side you can set a scrape config as follows
 
         - job_name: mysql # To get metrics about the mysql exporter’s targets
+          metrics_path: /probe
           params:
             # Not required. Will match value to child in config file. Default value is `client`.
             auth_module: [client.servers]