fix typo in main/build.html

[eltociear]

arbitary -> arbitrary

[vsoch]

Thank you for catching this! I think we'd want to fix it here:

singularity-cli/spython/main/build.py

Line 45 in 1daab24

image: the image to build (if None, will use arbitary name

and then run the docs generation via this script: https://github.com/singularityhub/singularity-cli/blob/master/.docs/apidoc.sh.

[vsoch]

I'll put in a PR tonight to fix this up, and also add a spellchecker to avoid future issues. Thanks!