Correct example PowerShell -Uri argument name (#2304)

The Getting Started page has sample PowerShell that readers can execute
to integrate with Neovim. However, the sample code has a typo, with the
hyphen missing for the -Uri argument.

I've corrected the typo.

This is the error that readers would encounter before this fix:

> Invoke-WebRequest -Method 'GET' Uri $DownloadUrl -OutFile $ZipPath;
Invoke-WebRequest: A positional parameter cannot be found that accepts argument 'https://github.com/neovim/nvim-lspconfig/archive/refs/heads/master.zip'.

Co-authored-by: Bart Joy <bart@ecargo.co.nz>

Author: nzbart

docs/guide/getting_started.md

@@ -45,7 +45,7 @@ Alternatively, you can extract the zip file into the same place:
 $DownloadUrl = 'https://github.com/neovim/nvim-lspconfig/archive/refs/heads/master.zip';
 $ZipPath = "$HOME/AppData/local/nvim/nvim-lspconfig.zip";
 $InstallPath = "$HOME/AppData/local/nvim/pack/complete/start/nvim-lspconfig";
-Invoke-WebRequest -Method 'GET' Uri $DownloadUrl -OutFile $ZipPath;
+Invoke-WebRequest -Method 'GET' -Uri $DownloadUrl -OutFile $ZipPath;
 Expand-Archive -Path $ZipPath -DestinationPath $InstallPath;
 ```