Merge #34

34: Added missing link to edition 2018 guide r=therealprof a=caemor

It looked like the link to the edition guide was missing. But it might have been intentional because the edition guide might be moved to doc.rust-lang.org later on (rust-lang/edition-guide#76) :-)

Co-authored-by: Chris <11088935+caemor@users.noreply.github.com>

src/preface.md

@@ -42,7 +42,7 @@ need to install the following tools to run and inspect the examples in this
 book:
 
 - All the code in this book uses the 2018 edition. If you are not familiar with
-  the 2018 features and idioms check [the edition guide].
+  the 2018 features and idioms check the [`edition guide`].
 
 - Rust 1.31 or a newer toolchain PLUS ARM Cortex-M compilation support.
 
@@ -55,6 +55,8 @@ book:
 
 - GDB with ARM support.
 
+[`edition guide`]: https://rust-lang-nursery.github.io/edition-guide/
+
 ### Example setup
 
 Instructions common to all OSes