Enable rtt in embed for chapter 11

[lbellomo]

Reading 11-uart/receive-a-single-byte I found that the examples/receive-byte.rs don't work (compile, flash, but don't start the rtt) because Embed.toml in that chapter has the rtt disabled.

It's should be:

[default.general]
chip = "nrf52833_xxAA"

[default.reset]
halt_afterwards = false

[default.rtt]
enabled = true

[default.gdb]
enabled = false

And this should be the same for the rest of the examples in this chapter.