Instructions for using fzf installed through apt

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- [x] Neovim

I installed fzf through apt (following instructions for Debian) and can't seem to get fzf.vim to recognise it.

In my `.vimrc`:

```vim
Plug '/usr/bin/fzf'
Plug 'junegunn/fzf.vim'
```

fzf location:
```sh
$ where fzf
/usr/bin/fzf
```

My vim version
```sh
$ vim --version
NVIM v0.3.4
Build type: Release
LuaJIT 2.1.0-beta3
Compilation: /usr/bin/cc -g -O2 -fdebug-prefix-map=/build/neovim-RiUqEm/neovim-0.3.4=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=1 -DDISABLE_LOG -Wdate-time -D_FORTIFY_SOURCE=1 -Wconversion -O2 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fdiagnostics-color=auto -Wno-array-bounds -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -I/build/neovim-RiUqEm/neovim-0.3.4/build/config -I/build/neovim-RiUqEm/neovim-0.3.4/src -I/usr/include -I/build/neovim-RiUqEm/neovim-0.3.4/build/src/nvim/auto -I/build/neovim-RiUqEm/neovim-0.3.4/build/include
Compiled by team+vim@tracker.debian.org

Features: +acl +iconv -jemalloc +tui
See ":help feature-compile"

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/usr/share/nvim"

Run :checkhealth for more info

```

When I type `:Rg` in vim, I get the following:
```
Error detected while processing function fzf#vim#grep[22]..77_fzf[18]..77_wrap:
line 9:
E117: Unknown function: fzf#wrap
E15: Invalid expression: fzf#wrap(a:name, opts, a:bang)
line 10:
E121: Undefined variable: wrapped
line 14:
E121: Undefined variable: wrapped
E15: Invalid expression: wrapped
Error detected while processing function fzf#vim#grep[22]..77_fzf:
line 18:
E117: Unknown function: fzf#run
E15: Invalid expression: fzf#run(s:wrap(a:name, merged, bang))
Press ENTER or type command to continue
```

From other issues I understand that this means that fzf.vim can't find fzf. I also tried using the `rtp` variable, but to no avail.

Thanks