docs(api): Use correct HTTP method for specific issue (#13854)

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## DESCRIBE YOUR PR
While going through the API docs page, I noticed that the example around
mutating issues uses the POST method, but earlier on this docs page, it
explicitly states that any endpoints whose purpose is to just update
should be a PUT. Moreover, the endpoints in this example are real and
there is no POST for them.

## IS YOUR CHANGE URGENT?  
No

Author: NicoHinderling

develop-docs/backend/api/design.mdx

@@ -119,24 +119,24 @@ POST /resources/{id}
 
 ### Batch Operations
 
-Resources can get complicated when you need to expose batch operations vs single resource operations. For batch operations it is preferred to expose them as a `POST` request on the collection when possible.
+Resources can get complicated when you need to expose batch operations vs single resource operations. For batch operations it is preferred to expose them as a `PUT` request on the collection when possible.
 
 Let's say for example we have an endpoint that mutates an issue:
 
 ```
-POST /api/0/organizations/{org}/issues/{issue}/
+PUT /api/0/organizations/{org}/issues/{issue}/
 ```
 
 When designing a batch interface, we simply expose it on the collection instead of the individual resource:
 
 ```
-POST /api/0/organizations/{org}/issues/
+PUT /api/0/organizations/{org}/issues/
 ```
 
 You may also need to expose selectors on batch resources, which can be done through normal request parameters:
 
 ```
-POST /api/0/organizations/{org}/issues/
+PUT /api/0/organizations/{org}/issues/
 {
   "issues": [1, 2, 3]
 }
@@ -237,7 +237,7 @@ Expanding responses allow us to include relational information on a resource wit
 
 In general, endpoints should expose the fewest fields that will make the API usable in the general scenario. Doing one SQL request per API request is a good rule of thumb. To return information on a bounded relationship, endpoints should rely on the `expand` parameter. To return an unbounded relationship, it should be another endpoint.
 
-To take an example, let's talk about the projects list endpoint. A project belongs to an organizations but could be on multiple teams.
+To take an example, let's talk about the projects list endpoint. A project belongs to an organization but could be on multiple teams.
 
 By default, here's what the project endpoint should look like