Add additional slash to show ReceivePayjoin cli docs

[nymius]

Description

A missing slash was preventing the receive_payjoin command from displaying its documentation string:

Wallet operations.

...
  sync               Syncs with the chosen blockchain server
  broadcast          Broadcasts a transaction to the network. Takes either a raw transaction or a PSBT to extract
  receive_payjoin
  send_payjoin       Sends an original PSBT to a BIP 21 URI and broadcasts the returned Payjoin PSBT
  new_address        Get a new external address
...

This change adds the additional slash to make a proper doc string, so clap displays it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.86%. Comparing base (a675935) to head (a6d22b9).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #253   +/-   ##
=======================================
  Coverage   10.86%   10.86%           
=======================================
  Files           8        8           
  Lines        2466     2466           
=======================================
  Hits          268      268           
  Misses       2198     2198           

Flag	Coverage Δ
rust	10.86% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tvpeter]

ACK a6d22b9