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/src/conv.c b/src/conv.c
index ebc3eda..f13deef 100644
--- a/src/conv.c
+++ b/src/conv.c
@@ -27,7 +27,7 @@
  */
 
 /*! \file conv.c
- *  \file Osmocom convolutional encoder and decoder
+ * Osmocom convolutional encoder and decoder
  */
 #include "config.h"
 #ifdef HAVE_ALLOCA_H