Apply suggestions from code review

Co-authored-by: Xiaodi Wu <[email protected]>

Author: NevinBR

stdlib/public/core/Sort.swift

@@ -402,11 +402,11 @@ internal func _merge<Element>(
     //
     // Storage: [4, 4, 7, 8, 9, 16, x, x,  x,  x,  x]
     //                              ^                 ^
-    //                       srcHigh/destLow        destHigh (past the end)
+    //                       srcHigh/destLow       destHigh (past the end)
     //
     // Buffer:                     [8, 8, 10, 12, 15, x, ...]
     //                              ^                 ^
-    //                           bufferLow        bufferHigh
+    //                          bufferLow         bufferHigh
     buffer.moveInitialize(from: mid, count: highCount)
     bufferHigh = bufferLow + highCount