mdoc(7) police: Fixed a few .Fa abuses.

lib/libc/stdtime/ctime.3

@@ -360,8 +360,10 @@ in the
 threaded environment.)
 .Pp
 The
-.Fa tm_zone
-field of a returned tm structure points to a static array of characters,
+.Va tm_zone
+field of a returned
+.Vt tm
+structure points to a static array of characters,
 which will also be overwritten by any subsequent calls (as well as by
 subsequent calls to
 .Xr tzset 3

lib/libc/sys/shmctl.2

@@ -89,8 +89,8 @@ or the process must have superuser privileges.
 .El
 .Pp
 The
-.Fa shmid_ds
-struct is defined as follows:
+.Vt shmid_ds
+structure is defined as follows:
 .\"
 .\" I fiddled with the spaces a bit to make it fit well when viewed
 .\" with nroff, but otherwise it's straight from sys/shm.h