add `quantity` body parameter to apidocs for "Purchase Gem or Gem-purchasable item"

[Alys]

In the API, the https://habitica.com/api/v3/user/purchase/:type/:key route allows you to specify the amount of items you wish to buy (e.g., more than one Hatching Potion at once, or more than one gold-purchasable Gem at once). You do that by supplying a body parameter called quantity, with the value being an integer indicating the amount of the item to buy.

This issue is to get the quantity option added to the apidoc page in this section: Purchase Gem or Gem-purchasable item

It should be added in a similar way to what's been done for the Purchase Hourglass-purchasable item route.

I'm labelling this as "good first issue". You can find the correct file to modify by doing git grep "Purchase Gem or Gem-purchasable item" in your local install.

[Alys]

I've added the "notify helpers" label to remind me to remove the tip about using quantity from Application Programming Interface > Errors in the API Documentation on the wiki.

[ATommasi]

I can take care of this