clarify command's behavior by adding dry run example

pages/common/convmv.md

@@ -2,6 +2,10 @@
 
 > Converts filenames from one encoding to another
 
-- convert encoding without dry run
+- convert encoding(dry run)
+
+`convmv -f {{from_encoding}} -t {{to_encoding}} {{input_file}}`
+
+- convert encoding(not dry run)
 
 `convmv -f {{from_encoding}} -t {{to_encoding}} --notest {{input_file}}`