fix tracing injection docs

Author: giortzisg

platform-includes/distributed-tracing/custom-instrumentation/go.mdx

@@ -27,10 +27,12 @@ For distributed tracing to work, the two headers `sentry-trace` and `baggage`, m
 You can generate this tracing information with the Sentry SDK's `hub.GetTraceparent()` and `hub.GetBaggage()` functions. Here's an example using Guzzle:
 
 ```go
-hub := sentry.CurrentHub()
+// get the request context
+ctx := r.Context()
+hub := sentry.GetHubFromContext(ctx)
 req, _ := http.NewRequest("GET", "http://exmaple.com", nil)
-req.Header.Add(sentry.SentryTraceHeader, hub.GetTraceparent())
-req.Header.Add(sentry.SentryBaggageHeader, hub.GetBaggage())
+req.Header.Set(sentry.SentryTraceHeader, hub.GetTraceparent())
+req.Header.Set(sentry.SentryBaggageHeader, hub.GetBaggage())
 ```
 
 In this example, tracing information is propagated to the project running at `https://example.com`. If this project uses a Sentry SDK, it will extract and save the tracing information for later use.