fix 'not enough arguments in call to searcher.IndexDocument'

[daidi]

not enough arguments in call to searcher.IndexDocument

[yowenter]

LGTM

demo 里直接运行 go run search_server.go 的确会报

command-line-arguments ./search_server.go:76: not enough arguments in call to searcher.IndexDocument

修改之后， 就 okay 了。