fix: change command 'er' to 'fx'

cmds/cmds.go

@@ -34,7 +34,7 @@ func init() {
 		"debug":        cmd_debug,
 		"essence":      cmd_essence,
 		"draw":         cmd_draw,
-		"er":           cmd_er,
+		"fx":           cmd_er,
 		"stock":        cmd_stock,
 		"crypto":       cmd_crypto,
 	}

cmds/er.go

@@ -20,7 +20,7 @@ type ExchangeRateResponse struct {
 
 func cmd_er(c *qbot.Client, msg *qbot.Message, args *ArgsList) {
 	if args.Size < 3 {
-		c.SendMsg(msg, "用法: er <源币种> <目标币种>\n例如: er CNY HKD")
+		c.SendMsg(msg, "用法: fx <源币种> <目标币种>\n例如: fx CNY HKD")
 		return
 	}