Replace api.backdropcms.org with docs.backdropcms.org

[avpaderno]

Since the domain name for the documentation sub-site has been changed, links referring to that sub-site should be changed to use the docs.backdropcms.org domain name instead of api.backdropcms.org.

api.backdropcms.org is actually used just only in the README.md file. The other file containing a link that used the api.backdropcms.org domain has been fixed in #17.

[avpaderno]

(I did not wait seven days to merge this PR because I had to fix a typo on my previous commit. I apologize for the mistake.)