[mlir][NFC] Fix file comments.

Differential Revision: https://reviews.llvm.org/D81858

mlir/lib/Dialect/StandardOps/Transforms/ExpandAtomic.cpp

-//===- ExpandAtomic.cpp - Code to perform loop fusion ---------------------===//
+//===- ExpandAtomic.cpp - Code to perform expanding atomic ops ------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.