[NFC] Correct comment: exponentially -> quadratically

[tlongeri]

Probing is at hash, hash + 1, hash + 1 + 2, hash + 1 + 2 + 3... as can be seen in SmallPtrSetImplBase::FindBucketFor

llvm/include/llvm/ADT/SmallPtrSet.h

 //===- llvm/ADT/SmallPtrSet.h - 'Normally small' pointer set ----*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 ///

 /// Small sets use an array of pointers allocated in the SmallPtrSet object,
 /// which is treated as a simple array of pointers.  When a pointer is added to
 /// the set, the array is scanned to see if the element already exists, if not
 /// the element is 'pushed back' onto the array.  If we run out of space in the
 /// array, we grow into the 'large set' case.  SmallSet should be used when the
 /// sets are often small.  In this case, no memory allocation is used, and only
 /// light-weight and cache-efficient scanning is used.
 ///
-/// Large sets use a classic exponentially-probed hash table.  Empty buckets are
+/// Large sets use a classic quadratically-probed hash table.  Empty buckets are
 /// represented with an illegal pointer value (-1) to allow null pointers to be
 /// inserted.  Tombstones are represented with another illegal pointer value
 /// (-2), to allow deletion.  The hash table is resized when the table is 3/4 or
 /// more.  When this happens, the table is doubled in size.
 ///
 class SmallPtrSetImplBase : public DebugEpochBase {
   friend class SmallPtrSetIteratorImpl;