C++: Fix comment.

Author: geoffw0

cpp/ql/src/semmle/code/cpp/models/implementations/Allocation.qll

@@ -83,7 +83,7 @@ private class AllocaAllocationFunction extends AllocationFunction {
         // --- stack allocation
         "alloca", // // alloca(size)
         "__builtin_alloca", // __builtin_alloca(size)
-        "_alloca", // __alloca(size)
+        "_alloca", // _alloca(size)
         "_malloca" // _malloca(size)
       ]) and
     sizeArg = 0