style-guide: Fix example to match the rule it exemplifies (and match rustfmt)

An example immediately following "Put each bound on its own line." did
not put each bound on its own line.

Author: joshtriplett

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

@@ -369,7 +369,8 @@ where
         + Index<RangeTo<Idx>, Output = Self::Output>
         + Index<RangeFrom<Idx>, Output = Self::Output>
         + Index<RangeInclusive<Idx>, Output = Self::Output>
-        + Index<RangeToInclusive<Idx>, Output = Self::Output> + Index<RangeFull>,
+        + Index<RangeToInclusive<Idx>, Output = Self::Output>
+        + Index<RangeFull>,
 ```
 
 ### Type aliases