Update search_by_keyword.go

[faizanbashir]

Fixed error in the service.Search.List parameter as []string value is required.

The code returns the below error:

./search_by_keywords.go:33:30: cannot use "id,snippet" (untyped string constant) as []string value in argument to service.Search.List

I have update the parameter to a []string{"id, snippet"} which has fixed the error.

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