usage typo: Fix creating .minio.sys/background-ops bucket (#8957)

Due to a typo in the code, a cluster was not correctly creating
`background-ops` in all disks and nodes print the following error:

minio3_1  | API: SYSTEM()
minio3_1  | Time: 19:32:45 UTC 02/06/2020
minio3_1  | DeploymentID: d67c20fa-4a1e-41f5-b319-7e3e90f425d8
minio3_1  | Error: Bucket not found: .minio.sys/background-ops
minio3_1  |        2: cmd/data-usage.go:109:cmd.runDataUsageInfo()
minio3_1  |        1: cmd/data-usage.go:56:cmd.runDataUsageInfoUpdateRoutine()

This commit fixes the typo.

cmd/format-xl.go

@@ -643,7 +643,7 @@ func initXLMetaVolumesInLocalDisks(storageDisks []StorageAPI, formats []*formatX
 		// goroutine will return its own instance of index variable.
 		index := index
 		g.Go(func() error {
-			return makeFormatXLMetaVolumes(storageDisks[index])
+			return makeFormatXLMetaVolumes(disksToInit[index])
 		}, index)
 	}