Check for `/github` suffix

[JacobMGEvans]

Rather than compete with Discord Webhook docs there could be a check that appends if the /github suffix is missing from the Webhook being passed in.

action-discord/entrypoint.js

Line 36 in 5b40a0a

url = `${process.env.DISCORD_WEBHOOK}/github`;

[Ilshidur]

Note to self :
Refer to #98.