Update README.md. Fix power-shell command

[dmirgaleev]

Updated command for PowerShell.
Reasoning:

$fnm env --use-on-cd --shell powershell | Out-String | Invoke-Expression
error: invalid value 'powershell' for '--shell <SHELL>'
  [possible values: bash, zsh, fish, power-shell, cmd]

  tip: a similar value exists: 'power-shell'

For more information, try '--help'.
Invoke-Expression : Cannot bind argument to parameter 'Command' because it is an empty string.
At line:1 char:55
+ ... m env --use-on-cd --shell powershell | Out-String | Invoke-Expression
+                                                         ~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:PSObject) [Invoke-Expression], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyStringNotAllowed,Microsoft.PowerShell.Commands.InvokeExpressionCommand

fnm env --use-on-cd --shell power-shell | Out-String | Invoke-Expression worked fine. Tested on Windows 11

$PSVersionTable.PSVersion

Major  Minor  Build  Revision
-----  -----  -----  --------
5      1      22621  4391

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