Slightly snappier formulation (from code review)

Co-authored-by: lcnr <rust@lcnr.de>

Author: amandasystems

tests/ui/nll/relate_tys/placeholder-outlives-existential.rs

@@ -1,5 +1,4 @@
-// Test that the preprocessing step correctly handles some
-// cases of placeholder leaks.
+// Test that we correctly handle some cases of placeholder leaks.
 //
 //@ compile-flags:-Zno-leak-check