serial: docs: Fix filename for serial reference implementation

Currently documentation mentions a file but the filename has a typo in
it, underscore instead of hyphen.  This makes grep'ing for the file
hard.

Use correct filename for file amba-pl011.c

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>