[flang] Rename Mod-files.md to ModFiles.md

Original-commit: flang-compiler/f18@587a376f24a5d28abfdad20f3c7fda598f49f77d
Reviewed-on: https://github.com/flang-compiler/f18/pull/301
Tree-same-pre-rewrite: false

diff --git a/flang/documentation/Mod-files.md b/flang/documentation/ModFiles.md
similarity index 100%
rename from flang/documentation/Mod-files.md
rename to flang/documentation/ModFiles.md