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/serial.h b/include/osmocom/core/serial.h
index 889bd8a..1640a6d 100644
--- a/include/osmocom/core/serial.h
+++ b/include/osmocom/core/serial.h
@@ -25,7 +25,7 @@
  */
 
 /*! \file serial.h
- *  \file Osmocom serial port helpers
+ * Osmocom serial port helpers
  */
 
 #ifndef __OSMO_SERIAL_H__