Rollup merge of #81968 - pietroalbini:fix-doc-install-path, r=Mark-Simulacrum
Rollup merge of #81968 - pietroalbini:fix-doc-install-path, r=Mark-Simulacrum

bootstrap: fix wrong docs installation path

This PR fixes https://github.com/rust-lang/rust/issues/81967, a regression introduced by https://github.com/rust-lang/rust/pull/80797. The commit has already been backported to stable 1.50.0.

r? ``````@Mark-Simulacrum``````

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust