Fixed typo in README

The first example fails because there's no leading slash:

    $ go run main.go 
    panic: path must begin with '/'

    goroutine 1 [running]:
    runtime.panic(0x22f400, 0xc21000a660)
        /usr/local/Cellar/go/1.2.2/libexec/src/pkg/runtime/panic.c:266 +0xb6
    github.com/julienschmidt/httprouter.(*Router).Handle(0xc2100494e0, 0x2f6d20, 0x3, 0x309460, 0x4, ...)
        /Users/ysimonson/go/src/github.com/julienschmidt/httprouter/router.go:192 +0x6f
    github.com/gin-gonic/gin.(*RouterGroup).Handle(0xc210044200, 0x2f6d20, 0x3, 0x309460, 0x4, ...)
        /Users/ysimonson/go/src/github.com/gin-gonic/gin/gin.go:168 +0x3ce
    github.com/gin-gonic/gin.(*RouterGroup).GET(0xc210044200, 0x309460, 0x4, 0xc2100001b8, 0x1, ...)
        /Users/ysimonson/go/src/github.com/gin-gonic/gin/gin.go:178 +0x6d
    main.main()
        /Users/ysimonson/go/src/github.com/ysimonson/nutrino-api/main.go:9 +0xa1
    exit status 2