docs: update README.md

Author: thomasleplus

README.md

@@ -1,6 +1,6 @@
 # PDF
 
-Multi-platform Docker container with utilities to process PDF (`pdftk`, `ghostscript`, `ocrmypdf`, `pdfgrep`, `qpdf`...).
+Multi-platform Docker container with utilities to process PDF files (`pdftk`, `ghostscript`, `ocrmypdf`, `pdfgrep`, `qpdf`...).
 
 [![Dockerfile](https://img.shields.io/badge/GitHub-Dockerfile-blue)](pdf/Dockerfile)
 [![Docker Build](https://github.com/leplusorg/docker-pdf/workflows/Docker/badge.svg)](https://github.com/leplusorg/docker-pdf/actions?query=workflow:"Docker")