std.io.getStdOut() still appears in examples

[kchilton]

std.io.getStdOut() was replaced with std.fs.File.stdout(). This needs to be updated in:

• website/versioned_docs/version-0.15.x/02-standard-library/05-formatting.mdx
• website/versioned_docs/version-0.15.x/03-build-system/02-emitting-an-executable.md
• website/blog/fizz-buzz.md