key export --paper: fix restore hint for borg 2 syntax, fixes #10428

[ThomasWaldmann]

The borg key export --paper header told users to run borg key import --paper /path/to/repo, which is borg 1.x syntax. In borg 2, the positional argument of borg key import is the key backup path, and --paper rejects it.

The hint now says borg key import --paper. Like most of the docs, it assumes BORG_REPO is set. The expected output in key_cmds_test.py is updated to match.

Fixes #10428.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.50%. Comparing base (dd902f8) to head (10394b0).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10431      +/-   ##
==========================================
- Coverage   88.53%   88.50%   -0.04%     
==========================================
  Files         103      103              
  Lines       19148    19148              
  Branches     2978     2978              
==========================================
- Hits        16952    16946       -6     
- Misses       1532     1537       +5     
- Partials      664      665       +1     

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