doc/stdenv: fix typo in sourceRoot section (#339604)

If `unpackPhase` produces multiple source directories, you should set `sourceRoot` to the name of the intended directory.

You can also set `sourceRoot = ".";` if you want to control it yourself in a later phase.

For example, if your want your build to start in a sub-directory inside your sources, and you are using `fetchzip`-derived `src` (like `fetchFromGitHub` or similar), you need to set `sourceRoot = "${src.name}/my-sub-directory"`.

For example, if you want your build to start in a sub-directory inside your sources, and you are using `fetchzip`-derived `src` (like `fetchFromGitHub` or similar), you need to set `sourceRoot = "${src.name}/my-sub-directory"`.

##### `setSourceRoot` {#var-stdenv-setSourceRoot}