make variable names consistent in this example

..as suggested by Andre Schultz at OpusVL/perldoc.perl.org#57

Author: karenetheridge

pod/perlintro.pod

@@ -248,8 +248,8 @@ To get at hash elements:
 You can get at lists of keys and values with C<keys()> and
 C<values()>.
 
- my @fruits = keys %fruit_colors;
- my @colors = values %fruit_colors;
+ my @fruits = keys %fruit_color;
+ my @colors = values %fruit_color;
 
 Hashes have no particular internal order, though you can sort the keys
 and loop through them.