replace config.toml to bootstrap.toml in src:doc:unstable-book

Author: Shourya742

src/doc/unstable-book/src/compiler-flags/control-flow-guard.md

@@ -56,4 +56,4 @@ $Env:RUSTFLAGS = "-Z control-flow-guard"
 cargo +nightly build -Z build-std --target x86_64-pc-windows-msvc
 ```
 
-Alternatively, if you are building the standard library from source, you can set `control-flow-guard = true` in the config.toml file.
+Alternatively, if you are building the standard library from source, you can set `control-flow-guard = true` in the bootstrap.toml file.