commands: Use consistent style when describing subcommands

[jmooring]

This is a nit. I suggest that we:

• Use imperative mood
• Use sentence case
• Omit the trailing period

current

Available Commands:
  build       build builds your site
  completion  Generate the autocompletion script for the specified shell
  config      Print the site configuration
  convert     Convert your content to different formats
  deploy      Deploy your site to a Cloud provider.
  env         Print Hugo version and environment info
  gen         A collection of several useful generators.
  help        Help about any command
  import      Import your site from others.
  list        Listing out various types of content
  mod         Various Hugo Modules helpers.
  new         Create new content for your site
  server      A high performance webserver
  version     Print Hugo version and environment info

suggested

Available Commands:
  build       Build your site
  completion  Generate an autocompletion script for the specified shell
  config      Display site configuration
  convert     Convert front matter to another format
  deploy      Deploy your site to a cloud provider
  env         Display version and environment info
  gen         Generate documentation and syntax highlighting styles
  help        Display help about any command
  import      Import a site from another system
  list        List content
  mod         Manage modules
  new         Create new content
  server      Start the embedded web server
  version     Display version

Make similar changes to child subcommands.

[Hasaber8]

Not sure if it picked up yet or not, but I have raised an PR for it;
#12931