[flang] Rename directives.md to Directives.md

Original-commit: flang-compiler/f18@752d757e1ca0b437875ea5a2eddf3e0d18ff376d
Reviewed-on: https://github.com/flang-compiler/f18/pull/301
Tree-same-pre-rewrite: false

diff --git a/flang/documentation/directives.md b/flang/documentation/Directives.md
similarity index 100%
rename from flang/documentation/directives.md
rename to flang/documentation/Directives.md