Update stage instructions for the later stages for the streams extension

[libmartinito]

As pointed out by @LakshyaMittal3301, in the later stages, the instructions say that the response when a blocking read times out is a nil bulk string ($3\r\nnil\r\n). This is "nil" (a string), not a nil value.

The nil value representation of a bulk string is as follows $-1\r\n.

The stage instructions should be updated to reflect this.

[linear[bot]]

CC-1077 Update stage instructions for the later stages for the streams extension

[libmartinito]

Fixed by: codecrafters-io/build-your-own-redis#151