git_manuscript.md: Fix 'Github' -> 'GitHub' typo
git_manuscript.md: Fix 'uncertainly' -> 'uncertainty' typo
README.md: Fix 'github' -> 'GitHub' typo
README.md: the name of the system is 'Git', not 'git'

Following the Git commit 2de9b71 (Documentation: the name of the
system is 'Git', not 'git', 2013-01-21).
list_of_figures.md: the name of the system is 'Git', not 'git'
git_manuscript.md: the name of the system is 'Git', not 'git'
Merge pull request #21 from wking/typos

Fixes to assorted minor typos (git -> Git, uncertainly -> uncertainty, ...)