Update article_generate_context to article_generator_context

getpelican/pelican@f2d6f77462976f5ea291481cba40e041d42d9e8c changed
`article_generate_context` to `article_generator_context`. This commit
updates the plugin to reflect the change.
Merge pull request #61 from talha131/update-signal-article_generate_context

Update article_generate_context to article_generator_context