Fix PowerShell syntax issue in README

[jarrodsfarrell]

If there is a leading backslash wsl will fail with help-text.

Fixes #606

[jarrodsfarrell]

Though this seems to be something specific to wsl on Windows 10, my PowerShell version, or the copy of wsl I'm cursed with given @nzbr was able to run the command with a leading backslash. Double-checking by making sure ~\NixOS doesn't exist first will still cause wsl to fail to help-text if there's a leading backslash.

> wsl --version
WSL version: 2.3.26.0
Kernel version: 5.15.167.4-1
WSLg version: 1.0.65
MSRDC version: 1.2.5620
Direct3D version: 1.611.1-81528511
DXCore version: 10.0.26100.1-240331-1435.ge-release
Windows version: 10.0.19045.5247

> Get-ComputerInfo | Select-Object OSName, OSVersion, OsHardwareAbstractionLayer

OsName                   OsVersion  OsHardwareAbstractionLayer
------                   ---------  --------------------------
Microsoft Windows 10 Pro 10.0.19045 10.0.19041.5072

> $host.version

Major  Minor  Build  Revision
-----  -----  -----  --------
5      1      19041  5247

[nzbr]

Because you posted the PowerShell version, I also tried it in Windows PowerShell instead of PowerShell Core (which is the one I usually use) and it still works. Creating the directory before didn't break it either. But that might also be due to the updated WSL version

[nzbr]

Can you also change that in docs/src/install.md?

[jarrodsfarrell]

@nzbr Will do tomorrow (unless something comes up) since it's starting to seem like I need to clone the repo proper and edit the commit. There's some nuance that needs to be included as I'm founding out: microsoft/WSL#12383