Fix typo in notmuch.h documentation regarding database open modes

Reviewed-by: Carl Worth <cworth@cworth.org>:
The original proposal for having different open modes used the name
WRITABLE. I didn't like that name, (easy to misspell as WRITEABLE even
for native English speakers). So we renamed it to READ_WRITE
immediately, but apparently some of the documentation held the old
name for a while.