doc: update config file path in platform-support/wasm32-wasip1-threads.md

[iawia002]

The config content described in the Building the target section should be the configuration used for building Rust itself:

rust/config.example.toml

Lines 845 to 848 in 7d97c59

	# The root location of the `wasm32-wasip1` sysroot. Only used for WASI
	# related targets. Make sure to create a `[target.wasm32-wasip1]`
	# section and move this field there (or equivalent for the target being built).
	#wasi-root = <none> (path)

I believe this is different from Cargo's configuration. There seems to be some misunderstanding in the discussion here: #112922 (comment).

[rustbot]

r? @GuillaumeGomez

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[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Nilstrieb

[GuillaumeGomez]

r? Nilstrieb