Minor typos, grammar, and case sensitivity cleanup in HdfsPermissionsGuide.md's

Ran across a few minor grammatical/capitalization errors while reading through the HdfsPermissionsGuide in the Super-user section.

• my -> may
• my -> by
• lowercasing use of "Sticky" mid-sentence

Additionally, a few formatting/consistency issues:

• markdown formatting around kinit to match other command style
• "name node" -> NameNode to match the casing usage consistently