go.mod: change go version to 1.18

This will enable generics in examples for the slog handler guide.

Change-Id: I8fe2d876961899c15511226d29e1e5c2ef72519b
Reviewed-on: https://go-review.googlesource.com/c/example/+/509100
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

go.mod

diff --git a/go.mod b/go.mod
index c6b3f6b..80b7ee1 100644
--- a/go.mod
+++ b/go.mod
@@ -1,5 +1,5 @@
 module golang.org/x/example
 
-go 1.15
+go 1.18
 
 require golang.org/x/tools v0.0.0-20210112183307-1e6ecd4bf1b0