MINOR: [C++] Revert change in ipc/CMakeLists.txt (#41701)

### Rationale for this change

#40392 introduced a mistake in the the associated CMakeLists.txt, noticed by @ kou in #40392 (comment).

### What changes are included in this PR?

A reversion of that change.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>

cpp/src/arrow/ipc/CMakeLists.txt

@@ -6,7 +6,7 @@
 # "License"); you may not use this file except in compliance
 # with the License.  You may obtain a copy of the License at
 #
-# http://www.apache.org/licenses/LICENSE-2.0
+#   http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing,
 # software distributed under the License is distributed on an