Update csst_ms_sls2d.md

| keyword    | value                | comment                      | fallback_value | type | module               |

| ---------- | :------------------- | ---------------------------- | -------------- | ---- | -------------------- |

| CTYPE1     | 'RV---TPV'           |                              | 'RV---TPV'     | str  | csst_ms_sls_position |

| CTYPE1     | 'RA---TPV'           |                              | 'RA---TPV'     | str  | csst_ms_sls_position |

| CTYPE2     | 'DEC--TPV'           |                              | 'DEC--TPV'     | str  | csst_ms_sls_position |

| CUNIT1     | 'deg  '              |                              | 'deg'          | str  | csst_ms_sls_position |

| CUNIT2     | 'deg  '              |                              | 'deg'          | str  | csst_ms_sls_position |