Update git-p4 Perforce Script link to its new 301 URL home

The git-p4 Perforce Script cheese has moved to a new URL

Also fixes up a couple of new Unix (watch) Wikipedia links
missing the backslash on the parenthesis in the URL
Forgot that one Unix watch link missing the backslash
Merge pull request #164 from randomecho/perforce-kb-url-301

Classnotes fix: git-p4 Perforce Script URL went 301