Fix ID types to match JSON-RPC 2.0 spec (#569)

* Update index.mdx

updating description of id key according to [`JSON  RPC 2.0` standard](https://www.jsonrpc.org/specification#request_object)

* Update index.mdx

* Update docs/rpc/http/index.mdx

* Update index.mdx

Updating description based on specification

---------

Co-authored-by: Mike MacCana <mike.maccana@gmail.com>

docs/rpc/http/index.mdx

@@ -29,7 +29,9 @@ To make a JSON-RPC request, send an HTTP POST request with a
 fields:
 
 - `jsonrpc: <string>` - set to `"2.0"`
-- `id: <number>` - a unique client-generated identifying integer
+- `id: <string | number | null>` - a unique identifier for the request,
+  generated by the client. Typically a string or number, though null is
+  technically allowed but not advised
 - `method: <string>` - a string containing the method to be invoked
 - `params: <array>` - a JSON array of ordered parameter values