[ARM] Fix a test case typo. NFC.

The test looked for the wrong string, but it happened to match as
it was a substring of the actual output.

This fixes a typo from d8e67c1.

llvm/test/CodeGen/ARM/Windows/wineh-opcodes.ll

@@ -250,7 +250,7 @@ entry:
 ; CHECK-NEXT:         push.w  {r4, r5, r6, lr}
 ; CHECK-NEXT:         .seh_save_regs_w  {r4-r6, lr}
 ; CHECK-NEXT:         movw    r4, #0
-; CHECK-NEXT:         .seh_nop
+; CHECK-NEXT:         .seh_nop_w
 ; CHECK-NEXT:         movt    r4, #1
 ; CHECK-NEXT:         .seh_nop_w
 ; CHECK-NEXT:         bl      __chkstk