Update speedup figure in json-stream-rs-tokenizer README once buffering merged into json-stream

[smheidrich]

See daggaz/json-stream#47 (comment) - json-stream-rs-tokenizer README claim is ~4-10x, which will have to be reduced to ~3-4x once this is merged.

@SnoozeThis daggaz/json-stream#47

[SnoozeThis]

(https://snoozeth.is/wwA02OtdzH4) I will wait until daggaz/json-stream#47 is merged and then reassign to you.