[gardening] Fix a vs an typo

Author: practicalswift

lib/SILOptimizer/Transforms/ArrayElementValuePropagation.cpp

@@ -82,7 +82,7 @@ class ArrayAllocation {
     SILValue Replacement;
   };
 
-  /// Specifies the set of elements with which a append-contentof call can be
+  /// Specifies the set of elements with which an append-contentof call can be
   /// replaced.
   struct AppendContentOfReplacement {
     ApplyInst *AppendContentOfCall;