Minor Typo/Misspelling on Browsers and Devices docs page

Created by: tbolt

Very minor typo on this page: https://getbootstrap.com/docs/4.0/getting-started/browsers-devices/

Learn about the browsers and devices, from modern to told, that are supported by Bootstrap, including known quirks and bugs for each. "modern to told" should be "modern to old"

Happy to do a quick PR