Fix Net::SSH.start docs

[chooper]

Fixes the docs for Net::SSH.start's agent_socket_factory option.

Currently, the example code provided in the docs is not correct and this PR fixes that. Here is what's wrong with the current version:

• Net::SSH::start versus Net::SSH.start
• Args out of order for .start

Let me know if I need to do anything else to land this fix!

Cheers,
Charles

[codecov-io]

Codecov Report

Merging #561 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   95.71%   95.73%   +0.01%     
==========================================
  Files         142      142              
  Lines        8815     8855      +40     
==========================================
+ Hits         8437     8477      +40     
  Misses        378      378

Impacted Files	Coverage Δ
lib/net/ssh.rb	83.33% <ø> (ø)	⬆️
lib/net/ssh/test.rb	100% <0%> (ø)	⬆️
lib/net/ssh/test/kex.rb	100% <0%> (ø)	⬆️
test/connection/test_channel.rb	100% <0%> (ø)	⬆️
test/authentication/methods/test_hostbased.rb	100% <0%> (ø)	⬆️
test/authentication/methods/common.rb	100% <0%> (ø)	⬆️
...t/transport/kex/test_diffie_hellman_group1_sha1.rb	100% <0%> (ø)	⬆️
lib/net/ssh/verifiers/secure.rb	100% <0%> (ø)	⬆️
lib/net/ssh/transport/state.rb	100% <0%> (ø)	⬆️
test/authentication/test_agent.rb	100% <0%> (ø)	⬆️
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b9dabea...e437b30. Read the comment docs.