$font-family-body: 'Roboto', sans-serif;
$font-family-heading: 'Raleway', sans-serif;


h1, h2, h3, h4, h5, h6 {
    font-family: $font-family-heading;
    font-weight: medium;
}

h1 {
  font-size: 2.5rem;
}
h2 {
  font-size: 2rem;
}
h3 {
  font-size: 1.75rem;
}
h3 {
  font-size: 1.5rem;
}
h4 {
  font-size: 1.25rem;
}
h4 {
  font-size: 1rem;
}

p {
    font-family: $font-family-body;
}

$font-family-body: 'Roboto', sans-serif;
$font-family-heading: 'Raleway', sans-serif;


h1, h2, h3, h4, h5, h6 {
    font-family: $font-family-heading;
    font-weight: medium;
}

h1 {
  font-size: 2.5rem;
}
h2 {
  font-size: 2rem;
}
h3 {
  font-size: 1.75rem;
}
h3 {
  font-size: 1.5rem;
}
h4 {
  font-size: 1.25rem;
}
h5 {
  font-size: 1rem;
}

p {
    font-family: $font-family-body;
}

Update typo in bootstrap 4
Update typo in bootstrap 4

Seems there is a typo in bootstrap 4 alpha 6