Add role="tablist" to .nav-pills in examples to improve accessibility

Follow-up to e9374c0609c9e63fd6033f684b18ff105e1756ae
Per https://github.com/paypal/bootstrap-accessibility-plugin/commit/f203dea4b6d17264d7b776f02feaeeaa386cc9b2
Credit: https://github.com/paypal/bootstrap-accessibility-plugin & @rohk

[skip sauce]
Merge pull request #14070 from twbs/pills-role-tablist-too

Add role="tablist" to .nav-pills in examples to improve accessibility