Update sysadmin_101.rst

Improve some awkward phrasing/grammar
Update sysadmin_101.rst

Improve awkward phrasing
Update filesystems_101.rst

Correct misspelling.
Update package_management_101.rst

Correct misspellings. Comment about dependency resolution. Remove extraneous apostrophe. Insert omitted words.
Merge pull request #123 from dhutty/master

Correct some misspellings and the like. Minor rewordings.