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>

Documentation/serial/driver

@@ -7,7 +7,7 @@ This document is meant as a brief overview of some aspects of the new serial
 driver.  It is not complete, any questions you have should be directed to
 <rmk@arm.linux.org.uk>
 
-The reference implementation is contained within amba_pl011.c.
+The reference implementation is contained within amba-pl011.c.