[flang] Rename semantics.md to Semantics.md

Original-commit: flang-compiler/f18@98dd9af648ac0b8cc2329d901650919ce5a486f3
Reviewed-on: https://github.com/flang-compiler/f18/pull/301
Tree-same-pre-rewrite: false

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