docs: update output example to use valid checkstyle example; add json example (#3516)

.golangci.reference.yml

@@ -80,7 +80,7 @@ output:
   # Multiple can be specified by separating them by comma, output can be provided
   # for each of them by separating format name and path by colon symbol.
   # Output path can be either `stdout`, `stderr` or path to the file to write to.
-  # Example: "checkstyle:report.json,colored-line-number"
+  # Example: "checkstyle:report.xml,json:stdout,colored-line-number"
   #
   # Default: colored-line-number
   format: json