Fixed string literal error

[SulRash]

What does this PR do ?

Fixed the following string literal bug:

Traceback (most recent call last):
  File "/eph/nvme0/azureml/cr/j/5f8208feac9e43b3b90284ba713c821b/exe/wd/train_reward_model.py", line 22, in <module>
    from nemo_aligner.data.nlp.builders import (
  File "/opt/NeMo-Aligner/nemo_aligner/data/nlp/builders.py", line 45, in <module>
    from nemo_aligner.data.nlp.datasets import (
  File "/opt/NeMo-Aligner/nemo_aligner/data/nlp/datasets.py", line 346
    WARNING: It's recommended to preprocess your data in advance to ensure all samples are within self.seq_length.
               ^
SyntaxError: unterminated string literal (detected at line 346)

By changing It's to It is. This fix offers a wider range of compatibility without really modifying any of the logic.

Changelog

• Please update the CHANGELOG.md under next version with high level changes in this PR.

Changelog updated

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests? No necessary tests.
• Did you add or update any necessary documentation? Make sure to also update the NeMo Framework User Guide which contains the tutorials. No documentation update necessary.

[SulRash]

@gshennvm @odelalleau