Change __API_KEY__ to $API_KEY in API docs for consistency

api/README.md

@@ -73,7 +73,7 @@ POST /quotes
 ```
 
 ```BASH
-curl --request POST --header 'X-Quotes-API-Key: __API_KEY__' --data 'id=28&date=2018-01-01&game=IRL&text=Cheese?' 'https://ladylilia.com/quotes/api/quotes'
+curl --request POST --header 'X-Quotes-API-Key: $API_KEY' --data 'id=28&date=2018-01-01&game=IRL&text=Cheese?' 'https://ladylilia.com/quotes/api/quotes'
 ```
 
 *Note: the quote is replaced if it exists already*
@@ -92,7 +92,7 @@ PUT /quotes/:id
 ```
 
 ```BASH
-curl --request PUT --header 'X-Quotes-API-Key: __API_KEY__' --data 'date=2018-01-01&game=IRL&text=Cheese?' 'https://ladylilia.com/quotes/api/quotes/28'
+curl --request PUT --header 'X-Quotes-API-Key: $API_KEY' --data 'date=2018-01-01&game=IRL&text=Cheese?' 'https://ladylilia.com/quotes/api/quotes/28'
 ```
 
 *Note: the quote is created if it doesn't exist yet*
@@ -110,5 +110,5 @@ DELETE /quotes/:id
 ```
 
 ```BASH
-curl --request DELETE --header 'X-Quotes-API-Key: __API_KEY__' 'https://ladylilia.com/quotes/api/quotes/28'
+curl --request DELETE --header 'X-Quotes-API-Key: $API_KEY' 'https://ladylilia.com/quotes/api/quotes/28'
 ```