[ADT] Adjust a comment for one of the DenseMap constructors (NFC)

We now have:

  explicit SmallDenseMap(unsigned NumElementsToReservre = 0) {
    init(NumElementsToReservre);
  }

This patch adjusts the variable name in the DenseMap counterpart for
consistency and clarity.

Author: kazutakahirata

llvm/include/llvm/ADT/DenseMap.h

@@ -710,9 +710,11 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
   unsigned NumBuckets;
 
 public:
-  /// Create a DenseMap with an optional \p InitialReserve that guarantee that
-  /// this number of elements can be inserted in the map without grow()
-  explicit DenseMap(unsigned InitialReserve = 0) { init(InitialReserve); }
+  /// Create a DenseMap with an optional \p NumElementsToReservre to guarantee
+  /// that this number of elements can be inserted in the map without grow().
+  explicit DenseMap(unsigned NumElementsToReservre = 0) {
+    init(NumElementsToReservre);
+  }
 
   DenseMap(const DenseMap &other) : BaseT() {
     init(0);