docs: typo, serial.rs485 needs to be imported

fixes #159
diff --git a/serial/serialposix.py b/serial/serialposix.py
index 23e19c4..78959ce 100644
--- a/serial/serialposix.py
+++ b/serial/serialposix.py
@@ -52,7 +52,7 @@
 
 
 # some systems support an extra flag to enable the two in POSIX unsupported
-# partiy settings for MARK and SPACE
+# paritiy settings for MARK and SPACE
 CMSPAR = 0  # default, for unsupported platforms, override below
 
 # try to detect the OS so that a device can be selected...