dtrace_io.4: Use bio_length instead of bio_bcount in examples

Tracing bio_bcount makes little sense for some devices like for example
md(4), as it is set to "0" instead of to the actual I/O length.

markj@ suggested the following DTrace one-liner to identify some cases
where bio_length is set but bio_bcount is not:

    dtrace -n 'io:::start /args[0]->bio_length != args[0]->bio_bcount/{printf("%d %d", args[0]->bio_length, args[0]->bio_bcount); stack();}'

For future reference in the context of bio_length vs bio_bcount,
phk@ mentioned in the code review that:

> the original intent was to get rid of of bio_bcount

Reviewed by:	markj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D53365

share/man/man4/dtrace_io.4

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 18, 2015
+.Dd October 26, 2025
 .Dt DTRACE_IO 4
 .Os
 .Sh NAME
@@ -84,7 +84,7 @@ The following script shows a per-process breakdown of total I/O by disk device:
 
 io:::start
 {
-        @[args[1]->device_name, execname, pid] = sum(args[0]->bio_bcount);
+        @[args[1]->device_name, execname, pid] = sum(args[0]->bio_length);
 }
 
 END