Fix a spelling mistake, s/buiding/building

[huckphin]

Fixing a spelling mistake in first_concepts.html.

[lahosken]

Good catch! Alas, the source for this doc is in another repo; there instead of pull requests, we take changes as described at https://pantsbuild.github.io/howto_contribute.html

That last process is kind of heavyweight for a one-letter spelling fix if you're not a regular contributor :-) If you're thinking you might like to contribute to Pants in the future, then getting set up with that process is a good thing to do. If you're instead thinking "That escalated quickly, I don't want to deal with this," then rattle this PR a bit and someone else can make the change.

[huckphin]

Going through the process of submitting changes and getting setup for development sounds good to me. I am looking to help out where I can with the Pants project. I will go ahead and get setup for this PR to run through the motions and getting used to the code review process.

On a scale of 1 to 10, right now I am sitting at a solid 1, but would like to contribute where I can once I do get up to speed. I am really impressed with the technology, and am excited to learn more.

I can go ahead and close this PR, and will re-open through the correct process.