doc: fix link rot to `--keep-failed` in stdenv (#429167)

doc/stdenv/stdenv.chapter.md

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 - Other environment variables may be inconsistent with a `nix-build` either due to `nix-shell`'s initialization script or due to the use of `nix-shell` without the `--pure` option.
 
 If the build fails differently inside the shell than in the sandbox, consider using [`breakpointHook`](#breakpointhook) and invoking `nix-build` instead.
-The [`--keep-failed`](https://nixos.org/manual/nix/unstable/command-ref/conf-file#opt--keep-failed) option for `nix-build` may also be useful to examine the build directory of a failed build.
+The [`--keep-failed`](https://nixos.org/manual/nix/unstable/command-ref/conf-file#conf-keep-failed) option for `nix-build` may also be useful to examine the build directory of a failed build.
 :::
 
 ## Tools provided by `stdenv` {#sec-tools-of-stdenv}