Update doc for `SessionBuilder::control_directory`

[NobodyXu]

The doc is outdated and needs some update.

Signed-off-by: Jiahao XU Jiahao_XU@outlook.com

[jonhoo]

This change is 

[codecov-commenter]

Codecov Report

Merging #127 (7cedfe1) into master (3319a81) will decrease coverage by 1.07%. Report is 5 commits behind head on master. The diff coverage is 0.00%.

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Additional details and impacted files

| [Files Changed](https://app.codecov.io/gh/openssh-rust/openssh/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/builder.rs](https://app.codecov.io/gh/openssh-rust/openssh/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2J1aWxkZXIucnM=) | `76.57% <0.00%> (-7.03%)` | :arrow_down: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/openssh-rust/openssh/pull/127/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

[baoyachi]

LGTM.