Merge pull request brucefan1983#711 from brucefan1983/doc

fix stress convention

doc/nep/input_files/train_test_xyz.rst

@@ -39,8 +39,8 @@ Essentially any keyword is allowed, but we only read the following ones:
 * :attr:`energy=energy_value` such as :attr:`energy=-123.4` is mandatory and gives the target energy of the structure, which is :math:`-123.4` eV in this example.
 * :attr:`virial="vxx vxy vxz vyx vyy vyz vzx vzy vzz"` is optional and gives the :math:`3\times3` virial tensor of the structure in eV.
   Positive and negative values represent compressed and stretched states, respectively.
-* :attr:`stress="sxx sxy sxz syx syy syz szx szy szz"` is optional and gives the :math:`3\times3` stress tensor of the structure in eV/Å:math:`^3`.
-  Positive and negative values represent compressed and stretched states, respectively.
+* :attr:`stress="sxx sxy sxz syx syy syz szx szy szz"` is optional and gives the :math:`3\times3` stress tensor of the structure in eV/Å\ :math:`^3`.
+  Positive and negative values represent stretched and compressed states, respectively.
   If both :attr:`virial` and :attr:`stress` are present the former is used.
 * :attr:`weight=relative_weight` is optional and gives the relative weight for the current structure in the total loss function.
 * :attr:`properties=property_name:data_type:number_of_columns` is mandatory but only read the following items: