docs(lmp): fix the usage of LAMMPS pair_style hybrid/overlay (#4951)

As documented in https://docs.lammps.org/pair_hybrid.html, "In the
pair_coeff commands, the name of a pair style must be added after the
I,J type specification".

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- Documentation
- Updated the hybrid/overlay example for D3 dispersion to use
model-specific pair_coeff entries instead of placeholders.
- Clarifies how to declare coefficients for multiple models in one
configuration (e.g., Deep Potential and D3), improving accuracy and
reducing setup confusion.
- Example now better reflects real-world usage without altering existing
commands.
- No functional changes to the software; updates are limited to
documentation examples.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@ustc.edu.cn>

Author: njzjz

doc/third-party/lammps-command.md

@@ -319,6 +319,6 @@ For example, when `water.pb` is trained against the PBE0 functional, the simulat
 
 ```lammps
 pair_style hybrid/overlay deepmd water.pb dispersion/d3 original pbe0 30.0 20.0
-pair_coeff * * O H
-pair_coeff * * O H
+pair_coeff * * deepmd O H
+pair_coeff * * dispersion/d3 O H
 ```