Fix the docker example

[guedou]

After opening /bin/ls the architecture is switched to x86 by r2:

docker run --rm -it -e 'R2M2_ARCH=x86_64' guedou/r2m2
 -- Are you a wizard?
[0x00000000]> e~arch
anal.arch = r2m2
[..]
[0x00000000]> o /bin/ls
4
[0x0000487f]> e~arch
anal.arch = x86
[..]

Also, there is an OverflowError on the first call.