docs: change example for readableStreamFromReader and writableStreamFromWriter

[erfanium]

Currently doc examples suggests using readableStreamFromReader and writableStreamFromWriter to get a readableStream or writeableStream from a FsFile which is not required anymore after denoland/deno#13615

bad docs:

• https://deno.land/std@0.151.0/streams/mod.ts?s=readableStreamFromReader

[crowlKats]

Things can still just be a Deno.Reader or Deno.Writer, as one can just create an object with the required functions, so having readableStreamFromReader & writableStreamFromWriter still makes sense.

[erfanium]

@crowlKats Yeah, I'm now changing this issue to improve docs, because doc suggests readableStreamFromReader to read from FsFile

[lino-levan]

@kt3k with this being deprecated, we can probably close this issue.

[kt3k]

Thanks for the ping.

Closing as these APIs have been deprecated in #3640