Update link to SaltStack formula (prometheus#6024)

SuperQ · web-flow · commit 76769d4b6179 · 2019-09-15T16:59:16.000+02:00
Use the "official" saltstack-formulas version. The existing one hasn't
been updated since 2015.

Signed-off-by: Ben Kochie &lt;superq@gmail.com&gt;
diff --git a/docs/installation.md b/docs/installation.md
@@ -94,4 +94,4 @@ the following third-party contributions:
 
 ### SaltStack
 
-* [bechtoldt/saltstack-prometheus-formula](https://github.com/bechtoldt/saltstack-prometheus-formula)
+* [saltstack-formulas/prometheus-formula](https://github.com/saltstack-formulas/prometheus-formula)

Original file line number	Diff line number	Diff line change
`@@ -94,4 +94,4 @@ the following third-party contributions:`
`94`	`94`
`95`	`95`	`### SaltStack`
`96`	`96`
`97`		`-* [bechtoldt/saltstack-prometheus-formula](https://github.com/bechtoldt/saltstack-prometheus-formula)`
	`97`	`+* [saltstack-formulas/prometheus-formula](https://github.com/saltstack-formulas/prometheus-formula)`