Documentation fix where unsigned is used where signed is expected

Signed-off-by: Jordy Moos <jordymoos@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

doc/command.dox

@@ -56,7 +56,7 @@ If an error has been encountered, it returns a negative value:
 
 @code
   char buffer[256];
-  unsigned int nbytes;
+  int nbytes;
 
   nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
   while (nbytes > 0)

doc/guided_tour.dox

@@ -367,7 +367,7 @@ int show_remote_processes(ssh_session session)
   ssh_channel channel;
   int rc;
   char buffer[256];
-  unsigned int nbytes;
+  int nbytes;
 
   channel = ssh_channel_new(session);
   if (channel == NULL)
@@ -391,7 +391,7 @@ int show_remote_processes(ssh_session session)
   nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
   while (nbytes > 0)
   {
-    if (write(1, buffer, nbytes) != nbytes)
+    if (write(1, buffer, nbytes) != (unsigned int) nbytes)
     {
       ssh_channel_close(channel);
       ssh_channel_free(channel);