slog: make Info case in Level.String match the others
slog: make Info case in Level.String match the others

Since InfoLevel == 0, the code was correct.
But adding "- InfoLevel" makes it more obviously so.

Change-Id: I6b222fbae2bcc04d5e54a7319796c5e9347cf55d
Reviewed-on: https://go-review.googlesource.com/c/exp/+/447964
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/exp,golang/exp,golang/exp