Add trailing comma to match common style

Co-authored-by: Tshepang Mbambo <hopsi@tuta.io>

Author: ehuss

src/type-layout.md

@@ -132,7 +132,7 @@ The *`repr` [attribute][attributes]* can change the representation and layout of
 > struct ThreeInts {
 >     first: i16,
 >     second: i8,
->     third: i32
+>     third: i32,
 > }
 > ```
 
@@ -554,15 +554,15 @@ On their own, `align` and `packed` do not provide guarantees about the order of
 > struct PackedStruct {
 >     first: i16,
 >     second: i8,
->     third: i32
+>     third: i32,
 > }
 >
 > // C representation, alignment raised to 8
 > #[repr(C, align(8))]
 > struct AlignedStruct {
 >     first: i16,
 >     second: i8,
->     third: i32
+>     third: i32,
 > }
 > ```