fix grading period toggle button labeling

fixes CNVS-35437

test plan:
- go to accounts/1/grading_standards
- on the "Grading Periods" tab create multiple grading period sets
  with unique names
- using a SR, navigate to the toggle buttons on the grading periods
- note that the toggle button references the grading period name

Change-Id: Ic8727cef5df8248ab7d39ba3a1fb404d31d6161a
Reviewed-on: https://gerrit.instructure.com/105954
Tested-by: Jenkins
Reviewed-by: Steven Burnett <sburnett@instructure.com>
QA-Review: Deepeeca Soundarrajan <dsoundarrajan@instructure.com>
Product-Review: Felix Milea-Ciobanu <fmileaciobanu@instructure.com>

Author: Felix Milea-Ciobanu

app/jsx/grading/GradingPeriodSet.js

@@ -415,7 +415,7 @@ import 'jquery.instructure_misc_helpers'
               <div className="ItemGroup__header__title">
                 <button className={"Button Button--icon-action GradingPeriodSet__toggle"}
                         aria-expanded={this.props.expanded}
-                        aria-label="Toggle grading period visibility">
+                        aria-label={I18n.t('Toggle %{title} grading period visibility', { title: this.props.set.title })}>
                   <i className={"icon-mini-arrow-" + arrow}/>
                 </button>
                 <h2 ref="title" className="GradingPeriodSet__title">