slog: fix grammatical mistakes in docs

slog: fix grammatical mistakes in docs

Change-Id: I183dda71fcec04fc1e622c66f719bd745d8b4eea
Reviewed-on: https://go-review.googlesource.com/c/go/+/484975
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>