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/gsmtap_util.h b/include/osmocom/core/gsmtap_util.h
index 5609381..eaa32a0 100644
--- a/include/osmocom/core/gsmtap_util.h
+++ b/include/osmocom/core/gsmtap_util.h
@@ -54,4 +54,6 @@
 		int8_t signal_dbm, uint8_t snr, const uint8_t *data,
 		unsigned int len);
 
+/*! @} */
+
 #endif /* _GSMTAP_UTIL_H */