Update association_basics.md

I think it's copy/paste typo :)
Merge pull request #51 from harrykiselev/patch-1

Update association_basics.md at [has_many :through] association
Update routing.md

podonok-style detected :)
Update routing.md

Merge same notes to single.
Merge pull request #53 from harrykiselev/patch-3

Update routing.md
images optimized for web views
update associations guide
update upgrading guide
update 4.0 release notes
update validations guide
update query guide
update layouts guide
typo fix