[wiki] Remove extra subdirectory

wiki/completions.md

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+## Shell Completions Troubleshooting
+[Shell completions are provided for ZSH, Bash, and PowerShell 7+](https://github.com/rwinkhart/MUTN/tree/main/completions).
+### ZSH completions not working?
+Make sure your ~/.zshrc contains the following (if sourcing from default completions location, usually `/usr/share/zsh/site-functions/_mutn`):
+```shell
+autoload -Uz compinit && compinit
+```
+...and then restart your shell.
+***
+### Bash completions not working?
+Install your distribution's `bash-completion` package or source the completion script manually.
+
+For most environments, manual sourcing means adding the following to your ~/.bashrc:
+```shell
+source /usr/share/bash-completion/completions/mutn
+```
+Note that this directory is different on FreeBSD.
+
+FreeBSD:
+```shell
+source /usr/local/share/bash-completion/completions/mutn
+```
+...and then restart your shell.
+
+*Please note that Bash completions are slightly more limited than ZSH completions, and as such, new entries will not be auto-completed until the completions script is re-sourced.*
+***
+### PowerShell completions not working?
+Ensure you are using modern PowerShell (7+) as opposed to the built-in Windows PowerShell.
+
+Currently, completions must be manually installed and sourced in your PowerShell profile as such:
+```shell
+Set-PSReadlineKeyHandler -Key Tab -Function Complete  # this line is optional; it makes tab completion function more similarly to Bash/ZSH
+. <drive letter>:\path\to\mutn.ps1
+```
+...and then restart your shell.