piem--write-mbox-to-maildir: Tweak error message

Most of the user-facing documentation says "Message-Id" to match the
form that git-format-patch and git-am use.  Do the same in
piem--write-mbox-to-maildir's error message.

1 files changed, 1 insertions, 1 deletions

diff --git a/piem.el b/piem.el
index e72f41b..47944e4 100644
--- a/piem.el
+++ b/piem.el
@@ -756,7 +756,7 @@ is used as the value of `browse-url-browser-function'."
                                      (save-excursion
                                        (message-narrow-to-head-1)
                                        (message-fetch-field "message-id" t)))
-                                   (error "Message lacks message-ID")))))))
+                                   (error "Message lacks Message-Id header")))))))
             (cl-incf n-skipped)
           (let ((case-fold-search nil))
             (while (re-search-forward