nrfiles vs nr_files mixups

The man page references nr_files, the real option is nrfiles.
Change man page and add nr_files alias to nrfiles.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fio.1 b/fio.1
index f01bf3a..b0276c4 100644
--- a/fio.1
+++ b/fio.1
@@ -226,7 +226,7 @@
 .BI size \fR=\fPint
 Total size of I/O for this job.  \fBfio\fR will run until this many bytes have
 been transfered, unless limited by other options (\fBruntime\fR, for instance).
-Unless \fBnr_files\fR and \fBfilesize\fR options are given, this amount will be
+Unless \fBnrfiles\fR and \fBfilesize\fR options are given, this amount will be
 divided between the available files for the job. If not set, fio will use the
 full size of the given files or devices. If the the files do not exist, size
 must be given.