go.dev/_content/blog: update build constraints in fuzz-beta post

CL 351569 updated this blog post to use gotip without arguments,
since the dev.fuzz branch has been merged to the main branch and
is slated to be included in Go 1.18.

Update the build constraint in the shown example accordingly,
since the gofuzzbeta constraint is no longer set as of CL 351118.
Suggest the use of a go1.18 build constraint instead.

While here, also update to the newer go install @latest
syntax for downloading the gotip command, for reasons
similar to what's described in CL 346091.

For golang/go#47037.
Updates golang/go#48080.

Change-Id: Icbf3d3ad5ef5532063ca3e093eed330f7ea0c0d9
Reviewed-on: https://go-review.googlesource.com/c/website/+/353430
Website-Publish: DO NOT USE <dmitshur@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>

Author: dmitshur

go.dev/_content/blog/fuzz-beta.md

@@ -29,7 +29,7 @@ for more details about this feature.
 
 To get started, you may run the following
 
-	$ go get golang.org/dl/gotip
+	$ go install golang.org/dl/gotip@latest
 	$ gotip download
 
 This builds the Go toolchain from the master branch. After running this, `gotip`
@@ -47,7 +47,8 @@ argument is passed to a `TestXxx` function.
 Below is an example of a fuzz target that’s testing the behavior of the [net/url
 package](https://pkg.go.dev/net/url#ParseQuery).
 
-	// +build gofuzzbeta
+	//go:build go1.18
+	// +build go1.18
 
 	package fuzz