Add missing code fence

Author: cherryblossom000

src/doc/style-guide/src/expressions.md

@@ -178,10 +178,12 @@ let f = Foo {
 Functional record update syntax is treated like a field, but it must never have
 a trailing comma. Do not put a space after `..`.
 
+```rust
 let f = Foo {
     field1,
     ..an_expr
 };
+```
 
 
 ### Tuple literals