put fits docco in summary

Author: d3v-null

mdbook/src/SUMMARY.md

@@ -52,6 +52,7 @@
   - [hyperdrive format](defs/source_list_hyperdrive.md)
   - [André Offringa (ao) format](defs/source_list_ao.md)
   - [RTS format](defs/source_list_rts.md)
+  - [FITS format](defs/source_list_fits.md)
 - [Calibration solutions file formats](defs/cal_sols.md)
   - [hyperdrive format](defs/cal_sols_hyp.md)
   - [André Offringa (ao) format](defs/cal_sols_ao.md)