Update function.md example.

The function is part of [filter]

Author: ubombi

docs/src/en/filter/function.md

@@ -14,6 +14,7 @@ To use the function feature, you only need to write a Lua script. After RedisSha
 
 Here's a specific example:
 ```toml
+[filter]
 function = """
 shake.log(DB)
 if DB == 0
@@ -144,4 +145,4 @@ end
 shake.call(DB, ARGV)
 ```
 
-The effect is to write source `db 1` to destination `db 2`, write source `db 2` to destination `db 1`, and leave other `db`s unchanged.
+The effect is to write source `db 1` to destination `db 2`, write source `db 2` to destination `db 1`, and leave other `db`s unchanged.