net/http/httputil: remove os.Stderr claim in ReverseProxy.ErrorLog docs
The motivation for doing so is to avoid making inaccurate claims.
Logging may not go to os.Stderr if anyone overrides the log package's
default output via https://godoc.org/log#SetOutput. Saying that
the standard logger is used should be sufficient to explain the
behavior, and users can infer that os.Stderr is used by default,
unless it's changed.
This change is the same as what was applied to http.Server.ErrorLog
documentation in CL 53950.