Document the current required toolchain version (#343)

Author: feikesteenbergen

doc/book.toml

@@ -6,4 +6,4 @@ src = "src"
 title = "PL/Rust Guide"
 
 [preprocessor.variables.variables]
-toolchain_ver = "1.67.1"
+toolchain_ver = "1.70.0"

doc/src/install-prerequisites.md

@@ -111,7 +111,7 @@ rustc -V
 The output from `rustc -V` should look similar to the following example.
 
 ```
-rustc {{toolchain_ver}} (d5a82bbd2 2023-02-07)
+rustc {{toolchain_ver}} (90c541806 2023-05-31)
 ```
 
 Use `rustup default` to check that the explicit version of `rustc` is