Update header text to match Twilio docs

Author: forstisabella

src/_sass/_variables.scss

@@ -154,7 +154,7 @@ $heading-font-weight: 500 !default;
 /// Font family of headings.
 ///
 /// @type {Keyword|Values}
-$heading-font-family: "Twilio Sans Display", "SF Pro Text", sans-serif !default;
+$heading-font-family: "Twilio Sans Text", "SF Pro Text", sans-serif !default;
 
 /// Color of headings.
 ///