luci-proto-gre: remove extra parenthesis

fix the following error:
SyntaxError
Unexpected token ')'
  in http://192.168.122.131/luci-static/resources/protocol/grev6.js:?
  at http://192.168.122.131/luci-static/resources/luci.js:22
  at async Promise.all (index 4)
  at async Promise.all (index 5)

Fixes: 2b7fd129

 ("luci-proto-gre: improvement of LuCI interface")
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>