doc/go1.19: minor fixes

doc/go1.19: minor fixes

EM_LONGARCH and R_LARCH_* are defined in package debug/elf. Change the
definition list title accordingly.

Format links sort.Find and sort.Search as code.

Add a link to syscall.Getrusage.

Change-Id: I30602baedda8ccac028101858a608f1d8ffb633b
Reviewed-on: https://go-review.googlesource.com/c/go/+/419214
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>