docs(README.md): several bugfix & improvement

[yuanminhui]

Description

Bug/typo fix & improvement in the deployment userguide.
Changes inside install-nfs.sh script is to achieve idempotence in writing nfs path config, since user may run the script several times due to potential problems.

Related Issue

Fixes Bio-OS/bioos/issues/32

Test

None.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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