version-control/git/remote/Readme.md: Fix 'upstreeam' -> 'upstream' typo
shell/Readme.md: use explicit URL (with https://)
fixed links

[wtk]:
This pulled in a few workshop-specific link changes, but I cherry
picked it to get the "GUI" -> "browser" update.
shell/Readme.md: Use USERNAME and YYYY-MM-PLACE in git clone instructions
Merge pull request #20 from wking/typos

Fix a few typos (e.g. `upstreeam` -> `upstream`)