Documentation fixes for AvoidUsingPlainTextForPassword.md

[avvi00]

Hello,

Some proposed fixes to the documentation:

• The recommended type in the How to Fix section has a typo.
• The 'wrong' example was using the correct approach (used SecureString).
• The 'wrong' example was inconsistent with the 'correct' example in terms of naming and spacing.
• Examples included extra parameters which (in my opinion) distracted from the point being conveyed.

regards,
Avner

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This change is

[msftclas]

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[msftclas]

@avvi00, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

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[kapilmb]

@avvi00 Thanks for the contribution.