fix prefix not start with /

[bensonfx]

Issue：

定义prefix 不以 / 开始时，生成openapi 2.0文档lint报错

@server(
    prefix: api/v1
    group: user
)
service loopx-api {
    @handler Login
    post /user/login (UserLoginReq) returns (UserInfoReply)
}

生成的openapi2.0

"api/v1/user/login": {
      "post": {
        "operationId": "Login",
        "responses": {
          "200": {
            "description": "A successful response.",
            "schema": {
              "$ref": "#/definitions/UserInfoReply"
            }
          }
        },
        "parameters": [
          {
            "name": "body",
            "in": "body",
            "required": true,
            "schema": {
              "$ref": "#/definitions/UserLoginReq"
            }
          }
        ],
        "tags": [
          "user"
        ]
      }
    },

[bensonfx]

@MaxToby pls review it.