fix(readme): we're actually merging from go1.19

I forgot to change the entry before.

Spotted while working on ooni/probe#2223.

README.md

@@ -227,7 +227,7 @@ git checkout main
 git remote add golang [email protected]:golang/go.git || git fetch golang
 git branch -D golang-upstream golang-http-upstream merged-main || true
 git fetch golang
-git checkout -b golang-upstream go1.18.3
+git checkout -b golang-upstream go1.19
 git subtree split -P src/net/http/ -b golang-http-upstream
 git checkout main
 git checkout -b merged-main