SDL Manual Correction

I remembered encountering an issue the first go through the manual, but
wasn't really aware of the community at that time.  I'm going back through
the most recent version and came across the following 'typo'.  I don't know
why, but I"m inclined to blame garu.

pg. 33 -> Remove the 'my' in line 51.  $brush_color is a global variable.

51 my $brush_color = $key_name if $key_name =~ /^nd$/;

The program will run, but the user will not be able to change the brush
color as long as 'my' is in there.