[flang] Rename intrinsics.md to Intrinsics.md

Original-commit: flang-compiler/f18@1d2bcb2183fdd290a2c96d784833b18873ec99a8
Reviewed-on: https://github.com/flang-compiler/f18/pull/301
Tree-same-pre-rewrite: false

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