README: remove Minicom comment

The main README file is the wrong place to document how different terminal
emulations can be used to access the U-Boot serial console. C-Kermit which
requires a configuration file or several commands to access the U-Boot
console may not be the preferred choice for newcomers. The provided wiki
link is invalid.

The man-pages for loadb and loads already show how to use the commands with
picocom.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>