fixed typo on API, line was repeated.

fixes CNVS-14227

test plan:
- visit /doc/api/assignments.html
- should only see one instance of line 'API for accessing assignment information.'

Change-Id: Ia0c55d2aaea4f7eeaefd30ea0427be75b0719b6a
Reviewed-on: https://gerrit.instructure.com/38465
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>