vimPlugins.vim-markdown-composer: fix executing
vimPlugins.vim-markdown-composer: fix executing

The source for setting the path is slightly different, so relax the
search/replace term, so it works with the new version.

Fixes starting the web-preview process.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs