doc: Fix a typo in curve25519-sha256@libssh.org.txt

Fix the typo 'stengths' -> 'strengths' and remove the trailing white
space on the same line.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

doc/curve25519-sha256@libssh.org.txt

@@ -22,7 +22,7 @@ claims to be able to eavesdrop. Having a secure replacement would make passive
 attacks much harder if such a backdoor exists.
 
 However an alternative exists in the form of Curve25519. This algorithm has been
-proposed in 2006 by DJB [Curve25519]. Its main stengths are its speed, its 
+proposed in 2006 by DJB [Curve25519]. Its main strengths are its speed, its
 constant-time run time (and resistance against side-channel attacks), and its
 lack of nebulous hard-coded constants.