fix: make --render work again

[acuteenvy]

This PR fixes the error when using --render.

$ tldr --render path/to/file
[...]
AttributeError: 'str' object has no attribute 'open'

[kbdharun]

LGTM, thanks for the fix.

---

Tested the changes locally quickly with WSL (after fix, before fix):

~/tldr-python-client# python3 tldr.py --render 1.md

  tldr

  Display simple help pages for command-line tools from the tldr-pages project.
  Note: the `--language` and `--list` options are not required by the client specification, but most clients implement them.
  More information: https://github.com/tldr-pages/tldr/blob/main/CLIENT-SPECIFICATION.md#command-line-interface.

  - Print the tldr page for a specific command (hint: this is how you got here!):
    tldr command

  - Print the tldr page for a specific subcommand:
    tldr command subcommand

  - Print the tldr page for a command in the given [L]anguage (if available, otherwise fall back to English):
    tldr --language language_code command

  - Print the tldr page for a command from a specific [p]latform:
    tldr --platform android|common|freebsd|linux|osx|netbsd|openbsd|sunos|windows command

  - [u]pdate the local cache of tldr pages:
    tldr --update

  - [l]ist all pages for the current platform and common:
    tldr --list

  - [l]ist all available subcommand pages for a command:
    tldr --list | grep command | column

~/tldr-python-client# tldr --render 1.md
Traceback (most recent call last):
  File "/snap/tldr/627/bin/tldr", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/snap/tldr/627/lib/python3.12/site-packages/tldr.py", line 665, in cli
    main()
  File "/snap/tldr/627/lib/python3.12/site-packages/tldr.py", line 610, in main
    with command.open(encoding='utf-8') as open_file:
         ^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'open'