misc: Doxygen tweaks: fixed some typos and minor errors

Doxygen generates quite a lot of warnings on libosmocore. Some of them
are obvious typos - this patch aims to fix such low-hanging fruit.
diff --git a/include/osmocom/core/conv.h b/include/osmocom/core/conv.h
index e5b2a97..4cc791f 100644
--- a/include/osmocom/core/conv.h
+++ b/include/osmocom/core/conv.h
@@ -25,7 +25,7 @@
  */
 
 /*! \file conv.h
- *  \file Osmocom convolutional encoder and decoder
+ * Osmocom convolutional encoder and decoder
  */
 
 #ifndef __OSMO_CONV_H__