docs: fix unknown configuration setting

Author: dzvon

docs/dev/README.md

@@ -98,7 +98,7 @@ After I am done with the fix, I use `cargo xtask install --client` to try the ne
 If I need to fix something in the `rust-analyzer` crate, I feel sad because it's on the boundary between the two processes, and working there is slow.
 I usually just `cargo xtask install --server` and poke changes from my live environment.
 Note that this uses `--release`, which is usually faster overall, because loading stdlib into debug version of rust-analyzer takes a lot of time.
-To speed things up, sometimes I open a temporary hello-world project which has `"rust-analyzer.withSysroot": false` in `.code/settings.json`.
+To speed things up, sometimes I open a temporary hello-world project which has `"rust-analyzer.cargo.noSysroot": true` in `.code/settings.json`.
 This flag causes rust-analyzer to skip loading the sysroot, which greatly reduces the amount of things rust-analyzer needs to do, and makes printf's more useful.
 Note that you should only use the `eprint!` family of macros for debugging: stdout is used for LSP communication, and `print!` would break it.