[Code scan] Fix malformed inline math roles in tutorials

[njzjz]

This issue comes from a Codex global scan of deepmodeling/tbplas at commit 4d3652b.

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:

tbplas/doc/source/tutorial/prim_cell/z2.rst

Line 182 in 4d3652b

To calculate :math:`\mathbb{Z}_2`` we need to sample :math:`\mathbf{k}_a` from

tbplas/doc/source/tutorial/prim_cell/z2.rst

Line 191 in 4d3652b

insulator. We then decrease the SOC intensity :math:`\lambda`` to 0.15 eV and re-calculate the

tbplas/doc/source/tutorial/prim_cell/model_complex.rst

Line 23 in 4d3652b

replicates the primitive cell along :math:`a`, :math:`b`, and :math:`c`` directions. :func:`apply_pbc`

tbplas/doc/source/tutorial/prim_cell/param_fit.rst

Line 16 in 4d3652b

cells, :math:`i` and :math:`\mathbf{k}`` are band and :math:`\mathbf{k}`-point indices,

tbplas/doc/source/tutorial/prim_cell/param_fit.rst

Line 204 in 4d3652b

:math:`V_{pp\sigma}` and :math:`V_{pp\pi}`` at first and second nearest hopping distances,

tbplas/doc/source/tutorial/advanced/hetero_model.rst

Line 150 in 4d3652b

:math:`4\times4\times1`` merged cells is created and plotted, with the hopping terms below 0.3 eV

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