[Demangle] fix comment NFC

[nickdesaulniers]

Summary

The second and third parameter of itaniumDemangle were removed in
commit 7277a72b908d ("[Demangle] remove unused params of itaniumDemangle")
Update a comment to reflect this.

[nathanchance]

Seems reasonable to me.

llvm/include/llvm/Demangle/Demangle.h

struct ItaniumPartialDemangler {
   ItaniumPartialDemangler &operator=(ItaniumPartialDemangler &&Other);
 
   /// Demangle into an AST. Subsequent calls to the rest of the member functions
   /// implicitly operate on the AST this produces.
   /// \return true on error, false otherwise
   bool partialDemangle(const char *MangledName);
 
   /// Just print the entire mangled name into Buf. Buf and N behave like the
-  /// second and third parameters to itaniumDemangle.
+  /// second and third parameters to __cxa_demangle.
   char *finishDemangle(char *Buf, size_t *N) const;
 
   /// Get the base name of a function. This doesn't include trailing template
   /// arguments, ie for "a::b<int>" this function returns "b".
   char *getFunctionBaseName(char *Buf, size_t *N) const;
 
   /// Get the context name for a function. For "a::b::c", this function returns
   /// "a::b".