docs(variables): wrap mut keyword with backtick

Author: guoard

exercises/variables/README.md

@@ -2,7 +2,7 @@
 
 In Rust, variables are immutable by default.
 When a variable is immutable, once a value is bound to a name, you can’t change that value.
-You can make them mutable by adding mut in front of the variable name.
+You can make them mutable by adding `mut` in front of the variable name.
 
 ## Further information