Update comment for forms mixins (#24050)

Check `form-control-focus` mixin, variable name is `$input-focus-border-color`

scss/mixins/_forms.scss

 // Form control focus state
 //
 // Generate a customized focus state and for any input with the specified color,
-// which defaults to the `@input-border-color-focus` variable.
+// which defaults to the `$input-focus-border-color` variable.
 //
 // We highly encourage you to not customize the default value, but instead use
 // this to tweak colors on an as-needed basis. This aesthetic change is based on