cabal: update description

Based on the first line of README.md

Author: MattSturgeon

nixfmt.cabal

@@ -3,8 +3,7 @@ name:                nixfmt
 version:             1.2.0
 synopsis:            Official formatter for Nix code
 description:
-  A formatter for Nix that ensures consistent and clear formatting by forgetting
-  almost all existing formatting during parsing.
+  Nixfmt is the official formatter for Nix language code.
 homepage:            https://github.com/NixOS/nixfmt
 bug-reports:         https://github.com/NixOS/nixfmt/issues
 license:             MPL-2.0