website: package-channel: Fix `vcs-file?' in channel section.

Building the package after the changes in section "Level 2: The repository as
a channel" would create an empty recursive copy since source files would be
above the current source directory (now `.guix/modules') passed to
`git-predicate', thus the resulting predicate would always be false for them.

Fixed by making the argument of `git-predicate' be the root of the Git
checkout again.

Also fix minor error in `package-with-configure-flags' documentation.

* website/drafts/package-channel.md: Fix `vcs-file?' in channel section.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

website/posts/package-channel.md

@@ -252,6 +252,11 @@ haven’t changed):
   #:use-module (guix git-download)   ;for ‘git-predicate’
   …)
 
+(define vcs-file?
+  ;; Return true if the given file is under version control.
+  (or (git-predicate (dirname (dirname (current-source-directory))))
+      (const #t)))                                ;not in a Git checkout
+
 (define-public guile
   (package
     (name "guile")
@@ -365,7 +370,7 @@ transformation options.  We can add them like so:
   …)
 
 (define (package-with-configure-flags p flags)
-  "Return P with FLAGS as addition 'configure' flags."
+  "Return P with FLAGS as additional 'configure' flags."
   (package/inherit p
     (arguments
      (substitute-keyword-arguments (package-arguments p)