Update custom-field-option.md

[drcoen]

Fixed minor typo: build() to buildForm()

[drcoen]

There's another reference to build() further down the page too, but I couldn't figure out how to edit my existing pull request in github, without having to download the repo!

[MolloKhan]

Thanks @drcoen I just applied the changes directly to the repository :)
d635000

[bocharsky-bw]

Thank you @drcoen for letting us know about this typo!

A tip on how to add more commits to your PR in the future: just click on drcoen:patch-2 link under the title of this PR. This will open the files in the current branch, where you can open a specific file, make tweaks in it and commit changes. I hope this helps!

Cheers!