doc/go1.19: fix typo: EM_LONGARCH -> EM_LOONGARCH

doc/go1.19: fix typo: EM_LONGARCH -> EM_LOONGARCH

Another last-minute fix. The whole repo is checked case-insensitively
against "longarch" and this is the only occurrence.

Change-Id: If13c123c1e44001a346f1df1df9d839c699703b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/420494
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>