docs(loader): remove duplicate "the"

Typo fixup

Closes #6521
docs(guide/expression): clarify
docs(guide/templates): clarity
docs(guide/services): rewrite services documentation
docs(guide/bootstrap): fix formatting
docs(guide/css-styling): rename and fix formatting
docs(guide/e2e-testing): rename and fix formatting
docs(guide/unit-testing): rename and fix formatting
docs(guide/introduction): reword for clarity
docs(guide/concepts): fix links
docs(guide/controller): reorganize, add Controller suffix, formatting
docs(guide/databinding): reword for clarity
docs(guide/di): fix formatting and improve clarity
docs(guide/module): improve clarity
docs(tutorial): update step_08.ngdoc

Closes #6537