Merge pull request #8207 from toph/log-json-file-example-format

Add an example json-file log line to the documentation.

config/containers/logging/json-file.md

@@ -13,6 +13,10 @@ and writes them in files using the JSON format. The JSON format annotates each l
 origin (`stdout` or `stderr`) and its timestamp. Each log file contains information about
 only one container.
 
+```json
+{"log":"Log line is here\n","stream":"stdout","time":"2019-01-01T11:11:11.111111111Z"}
+```
+
 ## Usage
 
 To use the `json-file` driver as the default logging driver, set the `log-driver`