Update ipc.rakudoc Demonstrating how to pipe data into a process using `:in`

[finanalyst]

The problem

proc (run/shell) is covered in a few places, but the way arbitrary data is piped into spawned process is very cryptic.

Solution provided

An example is provided that illustrates how to employ a terminal utility that is useful for web page design.

[coke]

Made it pass 'make xtest' in the branch. Looks good otherwise.