Fix documentation example for OpentelemetryProcessPropagator

The example calling `OpentelemetryProcessPropagator.fetch_parent_ctx` was passing the key to the `/1` arity function, which is not the argument it expects. To specify the key you also need to specify the depth first, using the `/2` arity function: `OpentelemetryProcessPropagator.fetch_parent_ctx(depth, key)`

propagators/opentelemetry_process_propagator/lib/opentelemetry_process_propagator.ex

@@ -50,7 +50,7 @@ defmodule OpentelemetryProcessPropagator do
     # you don't control, e.g. Ecto preloads
 
     Task.async(fn ->
-      parent_ctx = OpentelemetryProcessPropagator.fetch_parent_ctx(:"$callers")
+      parent_ctx = OpentelemetryProcessPropagator.fetch_parent_ctx(1, :"$callers")
 
       OpenTelemetry.Ctx.attach(parent_ctx)