fix sample-config for notmuch backend

This fixes the following error, as `subscribe` is not a boolean type.

> Config file contains errors: invalid type: boolean `true`, expected a
> string for key `accounts.notmuch.mailboxes.INBOX` at line 59 column 11

docs/samples/sample-config.toml

@@ -56,9 +56,9 @@
 #display_name = "Name Name"
 #  # notmuch mailboxes are virtual, they are defined by their alias and the notmuch query that corresponds to their content.
 #  [accounts.notmuch.mailboxes]
-#  "INBOX" = {  query="tag:inbox", subscribe = true }
-#  "Drafts" = {  query="tag:draft", subscribe = true }
-#  "Sent" = {  query="from:username@example.com from:username2@example.com", subscribe = true }
+#  "INBOX" = {  query="tag:inbox", subscribe = "true" }
+#  "Drafts" = {  query="tag:draft", subscribe = "true" }
+#  "Sent" = {  query="from:username@example.com from:username2@example.com", subscribe = "true" }
 #
 ## Setting up a Gmail account
 #[accounts."gmail"]