Fix relative links in the legacy BERT, Transformer and NLP docs

[pratikgx]

Description

Fixes six relative links that broke when the BERT and Transformer code moved under official/legacy/, so each one resolves on GitHub again.

• official/legacy/bert/README.md: the nlp/modeling link now points to ../../nlp/modeling, and the two data script links (and their visible paths) now point to ../../nlp/data/create_pretraining_data.py and ../../nlp/data/create_finetuning_data.py.
• official/legacy/transformer/README.md: beam_search.py is no longer in this folder. The link now points to official/nlp/modeling/ops/beam_search.py, which is the module transformer.py imports.
• official/nlp/docs/train.md: train.py is one level up, so the link is now ../train.py.
• official/README-TPU.md: the Transformer link now points to legacy/transformer.

Type of change

• Documentation update

Tests

Docs only, so no tests were run. Every new target exists on master.

Checklist

• I have signed the Contributor License Agreement.
• I have read guidelines for pull request.
• I have performed a self code review of my own code.
• I have made corresponding changes to the documentation.