Remove comment about nightly being necessary to build docs index.html

It isn't (anymore).

docs/rustdoc/README.md

@@ -2,9 +2,7 @@ This directory is empty except for the firebase config file we use to deploy
 rustdoc output, because the contents of the documentation are inline with the
 source code.  We use it to stage the rustdoc output for deployment in CI.
 
-The `index.html` can only be generated using nightly Rust, which we want to
-avoid in CI, because (1) it interferes with caching and (2) sometimes causes
-breakage.  When the list of built crates in the `deployments/scripts/rust-docs` script
+When the list of built crates in the `deployments/scripts/rust-docs` script
 changes, the `index.html` can be regenerated by setting `REGENERATE_INDEX=1`
 like so: