A Powershell cmdlet has a format of Verb-Noun. There should not be spaces between 'verb'(Get in this case),'-'and 'Noun'(help in this case).
PS C:\Users\Nitesh> Get-Help
TOPIC
Get-Help
SHORT DESCRIPTION
Displays help about Windows PowerShell cmdlets and concepts.
LONG DESCRIPTION
SYNTAX
get-help {<CmdletName> | <TopicName>}
help {<CmdletName> | <TopicName>}
<CmdletName> -?