fix: update drifted file:line citations in Go patterns

Upstream golang/go has shifted several line numbers since citations
were recorded. Updated 6 citations across 3 files:

- documentation.md: server.go:55-57 → 59-62 (ErrWriteAfterFlush)
- documentation.md: transport.go:79-80 → 72-73 (Transports should be reused)
- structs.md: client.go:31-35 → 30-34 (A Client is an HTTP client)
- structs.md: client.go:59-60 → 57-58 (type Client struct)
- style.md: stream.go:280 → 292 (var _ Marshaler)
- style.md: stream.go:280-281 → 292-293 (var _ Marshaler/Unmarshaler)

Verified against golang/go HEAD (depth=1 clone).

patterns/documentation.md

@@ -388,7 +388,7 @@ suggesting an alternative; using non-standard deprecation markers.
 **Code example from source:**
 
 ```go
-// net/http/server.go:55-57
+// net/http/server.go:59-62
 // Deprecated: ErrWriteAfterFlush is no longer returned by
 // anything in the net/http package. Callers should not
 // compare errors against this variable.
@@ -445,7 +445,7 @@ concurrent use by multiple goroutines").
 **Code examples from source:**
 
 ```go
-// net/http/transport.go:79-80
+// net/http/transport.go:72-73
 // Transports should be reused instead of created as needed.
 // Transports are safe for concurrent use by multiple goroutines.