dynamic-debug-howto: Correct echo -c to -n

Two of the example command lines use an argument to echo of "-c" which
isn't valid in (most versions of) echo causing these examples to fail.
Correct the argument to "-n" which works correctly.

Signed-off-by: Steven Price <steven@ecrips.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>