<fix>(para_split_v2): index out of range issue of span_text first char

[papayalove]

Motivation

fix a bug of index out of range issue of span_text first char in para_split_v2.py

Modification

fix a bug of index out of range issue of span_text first char in para_split_v2.py

Checklist

After PR:

• [ ] If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• [ ] CLA has been signed and all committers have signed the CLA in this PR.

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