binman: Fix up a format string in AssertInList()

Add the missing 's' to the required '%s' here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 48fec50..d0a8b75 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -395,7 +395,7 @@
         for grep in grep_list:
             if grep in target:
                 return
-        self.fail("Error: '%' not found in '%s'" % (grep_list, target))
+        self.fail("Error: '%s' not found in '%s'" % (grep_list, target))
 
     def CheckNoGaps(self, entries):
         """Check that all entries fit together without gaps