chore: change doc comment for quickstart command

[ghost]

Description

Adds the dfx quickstart to the dfx help menu.

Fixes # (issue)
https://dfinity.atlassian.net/browse/SDK-998

[ericswanson-dfinity]

Looks good (shortens the help text), but the bug in question seems to be out of date. The command does show help in dfx 0.15.1:

$ dfx --version
dfx 0.15.1
$ dfx quickstart --help
Use the `dfx quickstart` command to perform initial one time setup for your identity and/or wallet. This
command can be run anytime to repeat the setup process or to be used as an informational command, printing
information about your ICP balance, current ICP to XDR conversion rate, and more.

If setup tasks remain, this command is equivalent to running `dfx identity set-wallet` (for importing) or `dfx
ledger create-canister` followed by `dfx identity deploy-wallet` (for creating), though more steps may be
added in future versions.

Usage: dfx quickstart [OPTIONS]
...
$ dfx --help
The DFINITY Executor

Usage: dfx [OPTIONS] <COMMAND>

Commands:
...
  deps        Pull dependencies and integrate locally
  diagnose    Detects known problems in the current environment caused by upgrading DFX, and suggests commands to
                  fix them. These commands can be batch-run automatically via `dfx fix`
  fix         Applies one-time fixes for known problems in the current environment caused by upgrading DFX. Makes
                  no changes that would not have been suggested by `dfx diagnose`
  extension   Manages the dfx extensions
  generate    Generate type declarations for canisters from the code in your project
  identity    Manages identities used to communicate with the Internet Computer network. Setting an identity
                  enables you to test user-based access controls
  info        Get information about the replica shipped with dfx, path to networks.json, and network ports of
                  running replica
  ledger      Ledger commands
  new         Creates a new project
  ping        Pings an Internet Computer network and returns its status
  quickstart  Use the `dfx quickstart` command to perform initial one time setup for your identity and/or wallet.
                  This command can be run anytime to repeat the setup process or to be used as an informational
                  command, printing information about your ICP balance, current ICP to XDR conversion rate, and more
  remote      Commands used to work with remote canisters
  schema      Prints the schema for dfx.json
...
...

[ericswanson-dfinity]

These should probably all be // or ///, but not a mix

[ghost]

Thank you for reviewing Eric. I will close this PR as it looks like the issue is resolved in the latest.