Examples: use '.Get("query")' instead of '["query"][0]'

[WindomZ]

In examples,
if the query(RawQuery) values does not contain the key "query":

1. Use r.URL.Query()["query"][0], throws runtime error: index out of range.
2. Use r.URL.Query().Get("query"), gets the graphql.Result.Errors string such as [Must provide an operation.]. Maybe more friendly.

It works in go1.5.

[coveralls]

Coverage remained the same at 82.129% when pulling b2b9069 on WindomZ:master into 3e619b6 on graphql-go:master.

[chris-ramon]

lgtm, good catch!, thanks a lot @WindomZ 👍