docs(README, mod.ts): update code example

[otogawakatsutoshi]

The explanation of how to use mod.ts and READEME is outdated.
When I look at the implementation of the source code, I see getCrossOption, I see OsRecord, and I see a test called download_test.ts,
It does not expect the string "unix" to come in.

OsRecord is defined as below, though,

type OsRecord<T> = { [os in typeof Deno.build.os]: T };

When I look at https://deno.land/[email protected]?s=Deno.build of the document, the definition of "unix" is not present.
"unix" should be changed to linux.

[eliassjogreen]

I made the choice to write UNIX instead of Linux as there are multiple more non-linux posix compliant OSses but it might make more sense just to write Linux as to not confuse people.