http2/h2i: update install documentation

Change "go get" to "go install".

Change-Id: Ie62a4a24dfbfc53208df05845d03e64845eb41da
GitHub-Last-Rev: e1f9aef7b7c30ce9e187dde26fc5e30d73c75387
GitHub-Pull-Request: golang/net#138
Reviewed-on: https://go-review.googlesource.com/c/net/+/416015
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
diff --git a/http2/h2i/README.md b/http2/h2i/README.md
index fb5c5ef..4f72c76 100644
--- a/http2/h2i/README.md
+++ b/http2/h2i/README.md
@@ -25,7 +25,7 @@
 ## Installation
 
 ```
-$ go get golang.org/x/net/http2/h2i
+$ go install golang.org/x/net/http2/h2i@latest
 $ h2i <host>
 ```