fix: fix broken content-type

Author: foosel

pyproject.toml

@@ -6,7 +6,7 @@ build-backend = "setuptools.build_meta"
 name = "gcode-thumbnail-tool"
 description = "A small tool to analyse and extract thumbnails rendered into GCODE files"
 version = "0.1.0"
-readme = {file = "README.md", content-type = "markdown"}
+readme = {file = "README.md", content-type = "text/markdown"}
 
 authors = [
     {name = "Gina Häußge", email = "gina@octoprint.org"},