usage.dox: fix encoder/decoder subpage list indent

use \li to separate the list items contained in conditionals. this
avoids the encode page becoming a sub-item of decode; likely a problem
in <1.8.3.
+ fix encoder conditional, spelling error
+ correct encode page name to match decode 'Encoding'

Change-Id: I67890f52bed8e708bad63fb8819a074e0beff2ca

usage.dox

@@ -12,13 +12,13 @@
     - \ref usage_init
     - \ref usage_errors
 
-    Fore more information on decoder and encoder specific usage, see the
+    For more information on decoder and encoder specific usage, see the
     following pages:
     \if decoder
-    - \subpage usage_decode
+    \li \subpage usage_decode
     \endif
-    \if decoder
-    - \subpage usage_encode
+    \if encoder
+    \li \subpage usage_encode
     \endif
 
     \section usage_types Important Data Types