misc: Spelling and grammar fixes in comments

* it's -> its
* grammar fix in ui/vnc-enc-zywrle.h

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Don Koch <dkoch@verizon.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/slirp/if.c b/slirp/if.c
index 87ca8a5..fb7acf8 100644
--- a/slirp/if.c
+++ b/slirp/if.c
@@ -142,7 +142,7 @@
 
 /*
  * Send a packet
- * We choose a packet based on it's position in the output queues;
+ * We choose a packet based on its position in the output queues;
  * If there are packets on the fastq, they are sent FIFO, before
  * everything else.  Otherwise we choose the first packet from the
  * batchq and send it.  the next packet chosen will be from the session