Note on "Using Local Images in Embeds" in discord.py guide

[thomaspet]

We should add a note on the "Using Local Images in Embeds" section, stating that the filename parameter of discord.File should be the same as the end of the embed url (after attachment://)

Suggested by 609540346304659477 (RaVen) in #discord.py

[Den4200]

Updated to be more clear 👍