sync the stage0 filename

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/building/suggested.md

@@ -139,8 +139,7 @@ rustup override set nightly
 
 after [installing a nightly toolchain] with `rustup`. Don't forget to do this for all
 directories you have [setup a worktree for]. You may need to use the pinned
-nightly version from `src/stage0.json`, but often the normal `nightly` channel
-will work.
+nightly version from `src/stage0`, but often the normal `nightly` channel will work.
 
 **Note** see [the section on vscode] for how to configure it with this real rustfmt `x` uses,
 and [the section on rustup] for how to setup `rustup` toolchain for your bootstrapped compiler