Update doc/model/train-fitting-dos.md

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com> Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

Author: njzjz

doc/model/train-fitting-dos.md

@@ -159,7 +159,7 @@ dp --pt test -m frozen_model.pth -s ../data/111/$k -d ${output_prefix} -a -n 100
 
 ::::
 
-if `dp test -d ${output_prefix} -a` is specified, the predicted DOS and atomic DOS for each frame is output in the working directory
+if `dp test -d ${output_prefix} -a` is specified, the predicted DOS and atomic DOS for each frame are output in the working directory
 
 ```
 ${output_prefix}.ados.out.0 ${output_prefix}.ados.out.1 ${output_prefix}.ados.out.2 ${output_prefix}.ados.out.3