Grammar fix

Doc/lib/emailmessage.tex

@@ -34,7 +34,7 @@ The constructor takes no arguments.
 \end{classdesc}
 
 \begin{methoddesc}[Message]{as_string}{\optional{unixfrom}}
-Return the entire message flatten as a string.  When optional
+Return the entire message flattened as a string.  When optional
 \var{unixfrom} is \code{True}, the envelope header is included in the
 returned string.  \var{unixfrom} defaults to \code{False}.