fix typo in mnist.md: CheckpointReader -> CheckpointLoader (#64)
fix typo in mnist.md: CheckpointReader -> CheckpointLoader (#64)

* fix typo in mnist.md: CheckpointReader -> CheckpointLoader

mnottheone/deeplearnjs,mnottheone/deeplearnjs,mnottheone/deeplearnjs,mnottheone/deeplearnjs