Add missing Match to make the example self contained (fluent#1582)

Without the `Match` line nothing would be captured.

Signed-off-by: Stefan Sauer <st.kost+github@gmail.com>

Author: ensonic

pipeline/outputs/file.md

@@ -75,6 +75,7 @@ For example, if you set up the configuration as below:
 
 [OUTPUT]
   Name file
+  Match *
   Format template
   Template {time} used={Mem.used} free={Mem.free} total={Mem.total}
 ```