Update data_model.md

|  VER_INST | '0.0.1   '            | Version of instrument processing                | '0.0.1   '   | str  | csst_ms_mbi_instrument   |

|  STM_INST | '2022-12-30T10:18:53' | Time stamp of instrument processing             | ---          | str  | csst_ms_mbi_instrument   |

|  STA_INST | 0                     | 0=done 1=wrong                                  | 1            | i8   | csst_ms_mbi_distortion   | 

|  DATASUM  | '1352015684'          | *data unit checksum updated 2022-10-28T19:29:10 | ---          | str  | csst_ms_mbi_instrument   |

|  DATASUM  | '1352015684'          | data unit checksum                              | ---          | str  | csst_ms_mbi_instrument   |

#### Header of `csst_ms_mbi_distortion`

| NS_MATCH    | 25                    | total number of matched stars in 2 arcsec      | ---             | i8   | csst_ms_mbi_flux   |

| MED_CLR     | 0.0                   | median (BP-RP)_GAIA of matched stars           | -9999           | f32  | csst_ms_mbi_flux   |

| SKY_MAG     | 0.0359                | mag/arcsec^2                                   | -9999           | f32  | csst_ms_mbi_flux   |                 

| -SKY_RMS1   | 0.1766                | rms/pixel of the sky in unit of e-/s           | -9999           | f32  | csst_ms_mbi_flux   | 

| MAG_LIM     | 21.83                 | magnitude limiting of 5-sigma galaxy detection | -9999           | f32  | csst_ms_mbi_flux   | 

| STA_FLUX    | 0                     | flux calibration status                        | 1               | i8   | csst_ms_mbi_flux   |                       

| VER_FLUX    | '1.3     '            | version of calibration code                    | '1.3'           | str  | csst_ms_mbi_flux   |