Fix document location

Signed-off-by: Kohei Takahashi <flast@flast.jp>
More compliant original Range's documentation style

  * adaptors/adjacent_zipped
  * adaptors/dropped
  * adaptors/dropped_while
  * adaptors/elements
  * adaptors/elements_key
  * adaptors/memoized
  * adaptors/outdirected
  * adaptors/taken
  * adaptors/taken_while

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Adjust docs

Signed-off-by: Kohei Takahashi <flast@flast.jp>