escape backslashes in PassCmd examples

the config parser strips one level of backslash escapes.

src/mbsyncrc.sample

@@ -16,11 +16,11 @@ Pass xxxxxxxx
 # Fetch password from gnome-keyring:
 #PassCmd "gnome-keyring-query get mail_pw"
 # Fetch password from .netrc:
-#PassCmd "sed -n -e 's,^machine work\.host\.com login tehuser password \(.*\),\1,p' < $HOME/.netrc"
+#PassCmd "sed -n -e 's,^machine work\\.host\\.com login tehuser password \\(.*\\),\\1,p' < $HOME/.netrc"
 # Fetch password from a gpg-encrypted file:
 #PassCmd "gpg --quiet --for-your-eyes-only --decrypt $HOME/imappassword.gpg"
 # Fetch password from pwmd (http://pwmd.sourceforge.net/):
-#PassCmd "echo -ne 'GET myIsp\tpassword' | pwmc datafile"
+#PassCmd "echo -ne 'GET myIsp\\tpassword' | pwmc datafile"
 # On Mac OS X, run "KeyChain Access" -- File->New Password Item. Fill out form using
 #  "Keychain Item Name" http://IMAPSERVER  (note: the "http://" is a hack)
 #  "Account Name" USERNAME