Make rustup heading title match other titles

Author: mashuler

docs/user/manual.adoc

@@ -172,7 +172,7 @@ $ cargo xtask install --server
 If your editor can't find the binary even though the binary is on your `$PATH`, the likely explanation is that it doesn't see the same `$PATH` as the shell, see https://github.com/rust-lang/rust-analyzer/issues/1811[this issue].
 On Unix, running the editor from a shell or changing the `.desktop` file to set the environment should help.
 
-==== `rustup`
+==== rustup
 
 `rust-analyzer` is available in `rustup`: