Fix doxygen warning with usage.dox

Change-Id: I3ee8974a66f186fb0eb15b1078a3c7b9cbf5ec80

usage.dox

@@ -80,8 +80,11 @@
 
 
     The available initialization methods are:
-    \if encoder - #vpx_codec_enc_init (calls vpx_codec_enc_init_ver()) \endif
-    \if multi-encoder - #vpx_codec_enc_init_multi (calls vpx_codec_enc_init_multi_ver()) \endif
+    \if encoder
+     - #vpx_codec_enc_init (calls vpx_codec_enc_init_ver())
+     - #vpx_codec_enc_init_multi (calls vpx_codec_enc_init_multi_ver())
+     .
+    \endif
     \if decoder - #vpx_codec_dec_init (calls vpx_codec_dec_init_ver()) \endif