cmd/goimports: recommend go install instead of go get in docs

go get now logs a warning when using it to install globally. Prefer go
install @latest instead.

Change-Id: I9478771e70c58202847a6dfeaaf63e1b88301d84
Reviewed-on: https://go-review.googlesource.com/c/tools/+/355251
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Trust: Brad Fitzpatrick <[email protected]>

Author: ericchiang

cmd/goimports/doc.go

@@ -7,7 +7,7 @@
 Command goimports updates your Go import lines,
 adding missing ones and removing unreferenced ones.
 
-     $ go get golang.org/x/tools/cmd/goimports
+     $ go install golang.org/x/tools/cmd/goimports@latest
 
 In addition to fixing imports, goimports also formats
 your code in the same style as gofmt so it can be used