fio2gnuplot: Fixing typo when inserting fake data

Commit f9846c392d176582916c39b1925a436e79f23de1 introduced fake data
when matched files doesn't have the exact same amount of traces. This
patch introduced a typo that prevented the int comparison to work
properly.
diff --git a/tools/plot/fio2gnuplot.py b/tools/plot/fio2gnuplot.py
index 800c334..4be011d 100755
--- a/tools/plot/fio2gnuplot.py
+++ b/tools/plot/fio2gnuplot.py
@@ -162,7 +162,7 @@
 			s=file.readline().replace(',',' ').split()
 			if not s:
 				nb_empty_files+=1
-				s="-1, 0, 0, 0'".replace(',',' ').split()
+				s="-1, 0, 0, 0".replace(',',' ').split()
 
 			if (nb_empty_files == nb_files):
 				shall_break=True