Remove references to jj checkout

Fixes #3

src/typing-less-at-the-cli.md

@@ -5,8 +5,8 @@ long names, but you may want to take advantage of some of the aliases that
 exist. For example:
 
 ```console
-> jj checkout main
-> jj co main
+> jj status
+> jj st
 ```
 
 Same thing. But one of the coolest shortcuts is hard to represent here, because
@@ -29,10 +29,9 @@ you only need to share the amount of the ID that's unique in the repository to
 commands. So these are all equivalent:
 
 ```console
-> jj checkout main
-> jj co main
-> jj co 0262ab43
-> jj co 0
+> jj new main
+> jj new 0262ab43
+> jj new 0
 ```
 
 If there's another ID starting with `0` in the repository, it would show `02` in

src/working-with-github.md

@@ -256,13 +256,15 @@ if you made some changes right now? `HEAD` is still on our pull request branch,
 which we have deleted, right? `git` would have warned us about a detatched `HEAD`
 by deleting the branch (I hope, I think so, I didn't bother checking.)
 
-We can use `jj checkout` to move `@`:
+My `git` brain wants to do some sort of "check out `main` so that `@` is the tip
+of it," but that's not really the right way to think about it. There is a
+`jj checkout` because it is just a subset of `jj new`. After all, we aren't
+thinking in branches. We are thinking in changesets. If I want to start doing
+a new change on top of `main`, we should just say that:
 
 ```console
-> jj checkout main
-Working copy now at: pqtmqtnu 5c04adb7 (empty) (no description set)
-Parent commit      : uyovmwoq 41f8c46c main | (empty) Merge pull request #2 from steveklabnik/push-wzqkzmwoxpmn
+> jj new main
+Working copy now at: qqzrpmrs d2120f55 (empty) (no description set)
+Parent commit      : xmmnqkum 2a40cce2 some changes
 Added 0 files, modified 1 files, removed 0 files
 ```
-
-Now we're at a new change on top of `main`.