configure: fix enable-libopus help string

configure

@@ -192,7 +192,7 @@ External library support:
   --enable-libopencore-amrwb enable AMR-WB decoding via libopencore-amrwb [no]
   --enable-libopencv       enable video filtering via libopencv [no]
   --enable-libopenjpeg     enable JPEG 2000 de/encoding via OpenJPEG [no]
-  --enable-libopus         enable Opus decoding via libopus [no]
+  --enable-libopus         enable Opus de/encoding via libopus [no]
   --enable-libpulse        enable Pulseaudio input via libpulse [no]
   --enable-librtmp         enable RTMP[E] support via librtmp [no]
   --enable-libschroedinger enable Dirac de/encoding via libschroedinger [no]