[Code scan] Fix malformed inline math roles in tutorials

This issue comes from a Codex global scan of deepmodeling/tbplas at commit 4d3652b3dba8eb930e4869a6d41074f62d703692.

Severity: Low

Several RST tutorial lines close `:math:` roles with double backticks. That breaks inline math parsing in Sphinx and can render the surrounding text incorrectly.

Code references:
https://github.com/deepmodeling/tbplas/blob/4d3652b3dba8eb930e4869a6d41074f62d703692/doc/source/tutorial/prim_cell/z2.rst#L182
https://github.com/deepmodeling/tbplas/blob/4d3652b3dba8eb930e4869a6d41074f62d703692/doc/source/tutorial/prim_cell/z2.rst#L191
https://github.com/deepmodeling/tbplas/blob/4d3652b3dba8eb930e4869a6d41074f62d703692/doc/source/tutorial/prim_cell/model_complex.rst#L23
https://github.com/deepmodeling/tbplas/blob/4d3652b3dba8eb930e4869a6d41074f62d703692/doc/source/tutorial/prim_cell/param_fit.rst#L16
https://github.com/deepmodeling/tbplas/blob/4d3652b3dba8eb930e4869a6d41074f62d703692/doc/source/tutorial/prim_cell/param_fit.rst#L204
https://github.com/deepmodeling/tbplas/blob/4d3652b3dba8eb930e4869a6d41074f62d703692/doc/source/tutorial/advanced/hetero_model.rst#L150

Suggested fix: close each inline math role with a single backtick.

Start with the six referenced RST files under doc/source/tutorial, especially the cited lines in prim_cell/z2.rst, model_complex.rst, param_fit.rst, and advanced/hetero_model.rst. Check each reported inline math role and replace only the malformed closing double backticks with a single backtick. Done means all six references use valid inline math delimiters and the tutorials render without malformed math markup.

Written by the indexing model from the issue text.