Fix docs correct error handling in golang Headers & trailers section (#…

…198)

### Summary

This PR fixes a minor issue in the documentation's error handling
example. The original example incorrectly referenced `err` instead of
`connectErr` when accessing metadata, which could lead to confusion.

### Changes
- Updated the error handling example to correctly use
`connectErr.Meta().Get("Greet-Version")` instead of
`err.Meta().Get("Greet-Version")`.

Signed-off-by: s4s7 <[email protected]>
Co-authored-by: Sri Krishna <[email protected]>

docs/go/headers-and-trailers.md

@@ -75,7 +75,7 @@ func call() {
     connect.NewRequest(&greetv1.GreetRequest{}),
   )
   if connectErr := new(connect.Error); errors.As(err, &connectErr) {
-    fmt.Println(err.Meta().Get("Greet-Version"))
+    fmt.Println(connectErr.Meta().Get("Greet-Version"))
   }
 }
 ```