[RISCV][NFC] Adjust RISCVMoveMerge.cpp header to match standard style

* 80 columns
* Fix name of file (RISCVMoveMerger.cpp vs RISCVMoveMerge.cpp)
* `//===--` prefix rather than center-aligned text.

llvm/lib/Target/RISCV/RISCVMoveMerger.cpp

@@ -1,4 +1,4 @@
-//===---------- RISCVMoveMerge.cpp - RISCV move merge pass -------------===//
+//===-- RISCVMoveMerger.cpp - RISCV move merge pass -----------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.