Fix wrong properties's description

[kfirba]

Hey.

There are a few mistakes/typos I've found in the docs. What I found for now:

1. flexbox's align-items property where the .items-baseline class's property description is align-items: space-between:
   
2. background's background-color property where the .bg-* colors classes's description is using the color defenition:
   
3. background's background-size property where the .bg-cover and .bg-contain classes's description is using the background-position defenition:
   
4. border's border-color property where the border-{color} classes's description is using the color defenition:
   

This PR simply fixes that.

[reinink]

Wow, can you tell we were rushing a little near the end to get the docs out? 😂

Thanks so much @kfirba! ❤️

[kfirba]

@reinink It's all good :) You've done a splendid work!