Fix: bug in reading abacus md files

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 :tada:

Comparison is base (7bb6392) 82.69% compared to head (27e0a9d) 82.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #496 +/- ## ========================================== + Coverage 82.69% 82.71% +0.01% ========================================== Files 70 70 Lines 6398 6404 +6 ========================================== + Hits 5291 5297 +6 Misses 1107 1107 ``` | [Impacted Files](https://app.codecov.io/gh/deepmodeling/dpdata/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpdata/abacus/md.py](https://app.codecov.io/gh/deepmodeling/dpdata/pull/496?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBkYXRhL2FiYWN1cy9tZC5weQ==) | `99.21% <100.00%> (+0.03%)` | :arrow_up: |

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