Update answers_image_processing.rst

I modified the indexing in Step2.
Update index.rst

I couldn't find median_filter in module 'filter'.
I modified it to "filter.rank.median".
Merge pull request #117 from yasutomo57jp/patch-1

Update answers_image_processing.rst
Merge pull request #118 from yasutomo57jp/patch-2

Update index.rst