[Gardening] Tweaked comment grammar.

Author: nate-chandler

lib/SILOptimizer/SemanticARC/CopyValueOpts.cpp

@@ -172,7 +172,7 @@ bool SemanticARCOptVisitor::performGuaranteedCopyValueOptimization(
   // destroy_values. Check if:
   //
   // 1. All of our destroys are joint post-dominated by our end borrow scope
-  //    set. If they do not, then the copy_value is lifetime extending the
+  //    set. If they are not, then the copy_value is lifetime extending the
   //    guaranteed value, we can not eliminate it.
   //
   // 2. If all of our destroy_values are dead end. In such a case, the linear