fixed plural possessive

1 file owns = file's
2+ files owns = files'
English is strange sometimes

Author: tomByrer

readme.md

@@ -1,6 +1,6 @@
 # filesizemd
 
-> file's sizes as a markdown table (CLI)
+> files' sizes as a markdown table (CLI)
 
 [![Build](https://github.com/barelyhuman/filesizemd/actions/workflows/test.yml/badge.svg)](https://github.com/barelyhuman/filesizemd/actions/workflows/test.yml)