fix typo in doc atomic_v

Author: Yi-FanLi

doc/nep/input_parameters/atomic_v.rst

@@ -5,7 +5,7 @@
 :attr:`atomic_v`
 ================
 
-This keyword sets the mode :math:`\atomic_v` of whether to fit atomic or global quantities for dipole (`model_type = 1`) or polarizability (`model_type = 2`). Only one of atomic and global can be fitted at a time. Fitting both simultaneously is not supported. For the virial tensor (`model_type = 0`), only the global model is supported. 
+This keyword sets the mode `\atomic_v` of whether to fit atomic or global quantities for dipole (`model_type = 1`) or polarizability (`model_type = 2`). Only one of atomic and global can be fitted at a time. Fitting both simultaneously is not supported. For the virial tensor (`model_type = 0`), only the global model is supported. 
 The syntax is::
 
   atomic_v <mode>