filter KV: modify trimkey to trim_key

[answerwa]

    if trim_key, ok := config["trim_key"]; ok {
        plugin.trim_key = trim_key.(string)
    } else {
        plugin.trim_key = ""
    }

use 'trim_key' in source code 'filter/kv.go', but 'trimkey' in readme.md

[childe]

谢谢～