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/timer.c b/src/timer.c
index 98cbf6e..5988aef 100644
--- a/src/timer.c
+++ b/src/timer.c
@@ -128,7 +128,7 @@
 
 /*! \brief compute the remaining time of a timer
  *  \param[in] timer the to-be-checked timer
- *  \param[in] the current time (NULL if not known)
+ *  \param[in] now the current time (NULL if not known)
  *  \param[out] remaining remaining time until timer fires
  *  \return 0 if timer has not expired yet, -1 if it has
  *