trivial documentation correction for FIFO

[jmfriedt]

The documentation at https://gnss-sdr.org/docs/sp-blocks/signal-source/#reading-data-from-a-pipe states that the input arguments of the FIFO are sample_type defined as [ishort, gr_complex]. As I was about to implement the 8-bit integer data stream, I realized ibyte was implemented and seems to be working well. It would be nice to add this data type to the documentation, unless there is an issue I have not hit when using it.
Thanks