[Trivial] Fix Typo

[Trivial] Fix Typo

Fix Typo at
    modified:   nntrainer/dataset/data_iteration.h
    modified:   nntrainer/dataset/data_producer.h
    modified:   nntrainer/dataset/databuffer.h
    modified:   nntrainer/dataset/dir_data_producers.cpp
    modified:   nntrainer/dataset/random_data_producers.cpp
    modified:   nntrainer/layers/preprocess_l2norm_layer.h
    modified:   nntrainer/layers/split_layer.cpp

**Self evaluation:**
1. Build test:	 [X]Passed [ ]Failed [ ]Skipped
2. Run test:	 [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Donghak PARK <donghak.park@samsung.com>