Update rule-checking documentation to use "promtool".

content/docs/querying/rules.md

@@ -17,14 +17,12 @@ process. The changes are only applied if all rule files are well-formatted.
 
 ## Syntax-checking rules
 To quickly check whether a rule file is syntactically correct without starting
-a Prometheus server, install and run Prometheus's `rule_checker` tool:
+a Prometheus server, install and run Prometheus's `promtool` command-line
+utility tool:
 
 ```bash
-# If $GOPATH/github.com/prometheus/prometheus already exists, update it first:
-go get -u github.com/prometheus/prometheus
-
-go install github.com/prometheus/prometheus/tools/rule_checker
-rule_checker /path/to/example.rules
+go get github.com/prometheus/prometheus/cmd/promtool
+promtool check-rules /path/to/example.rules
 ```
 
 When the file is syntactically valid, the checker prints a textual