[avr-gcc-list] Typo in help message of avr-as 2.18

[Dmitry K.]

Hi.

'avr-as --help' (version 2.18) says:
  ...
  -mno-skip-bug   disable warnings for skipping two-word instructions
                  (default for avr4, avr5)

It is needed to add 'avr2' to the default list:
  -mno-skip-bug   disable warnings for skipping two-word instructions
                  (default for avr2, avr4, avr5)

Also correction (that avr2 is not a classic AVR) is needed in the
documentation to avr-as.

Test case:
  ; No warning, no errors with binutils-2.18:
  ; avr-as -mmcu=avr2
        .text
        .global main
  main: cpse    r0,r1
        lds     r2,0
        movw    r0,r2
        lpm     r2,Z+

Dmitry.