Docs: Custom icons docs need updating

[bilbof]

Under advanced usage > custom icons, this needs changing:

OhVueIcon.add is undefined.

Old:

OhVueIcon.add({
  name: 'bluesky-logo',
  width: 600,
  height: 530,
  d: 'm135.72 44.03c66.496 49.921 138.02 151.14 164.28 205.46 26.262-54.316 97.782-155.54 164.28-205.46 47.98-36.021 125.72-63.892 125.72 24.795 0 17.712-10.155 148.79-16.111 170.07-20.703 73.984-96.144 92.854-163.25 81.433 117.3 19.964 147.14 86.092 82.697 152.22-122.39 125.59-175.91-31.511-189.63-71.766-2.514-7.3797-3.6904-10.832-3.7077-7.8964-0.0174-2.9357-1.1937 0.51669-3.7077 7.8964-13.714 40.255-67.233 197.36-189.63 71.766-64.444-66.128-34.605-132.26 82.697-152.22-67.108 11.421-142.55-7.4491-163.25-81.433-5.9562-21.282-16.111-152.36-16.111-170.07 0-88.687 77.742-60.816 125.72-24.795z',
})

New:

addIcons({
  name: 'bluesky-logo',
  width: 600,
  height: 530,
  d: 'm135.72 44.03c66.496 49.921 138.02 151.14 164.28 205.46 26.262-54.316 97.782-155.54 164.28-205.46 47.98-36.021 125.72-63.892 125.72 24.795 0 17.712-10.155 148.79-16.111 170.07-20.703 73.984-96.144 92.854-163.25 81.433 117.3 19.964 147.14 86.092 82.697 152.22-122.39 125.59-175.91-31.511-189.63-71.766-2.514-7.3797-3.6904-10.832-3.7077-7.8964-0.0174-2.9357-1.1937 0.51669-3.7077 7.8964-13.714 40.255-67.233 197.36-189.63 71.766-64.444-66.128-34.605-132.26 82.697-152.22-67.108 11.421-142.55-7.4491-163.25-81.433-5.9562-21.282-16.111-152.36-16.111-170.07 0-88.687 77.742-60.816 125.72-24.795z',
})