Minor wording updates to the undefined behavior section

- Preserve the parallel between "unsafe traits and functions" at the beginning
  of the paragraph with "unsafe impls and blocks in the middle, by adding "impls
  and".

- Remove "even" for the condition of unsafe code being unsound at the end of the
  paragraph. With my understanding of English, "even when" means that the
  sentence holds even when (no pun intended) the "when" condition is removed,
  which is not true for that sentence. Sound unsafe code can trigger undefined
  behavior if its safety conditions are not upheld.

Author: ia0

src/undefined_behavior.md

@@ -159,9 +159,9 @@ arrangement of safe code can cause undefined behavior. That's a lot to consider!
 The "safety conditions" for unsafe traits and functions are just the conditions
 under which it does not trigger undefined behavior. These conditions aren't
 checked by the compiler, they're checked by the people who write the code
-itself. Therefore, unsafe blocks must be manually checked to verify that the
-code written upholds all of the conditions required to avoid undefined behavior.
-Any unsafe code that can trigger undefined behavior _even when its safety
+itself. Therefore, unsafe impls and blocks must be manually checked to verify
+that the code written upholds all of the conditions required to avoid undefined
+behavior. Any unsafe code that can trigger undefined behavior _when its safety
 conditions are upheld_ is unsound.