t/README: avoid poor-man's small caps GIT

t/README: avoid poor-man's small caps GIT

In 48a8c26c62 (Documentation: avoid poor-man's small caps GIT,
2013-01-21), the documentation was amended to spell Git's name as Git
when talking about the system as a whole. However, t/README was skipped
over when the treatment was applied.

Bring t/README into conformance with the CodingGuidelines by casing
"Git" properly.

While we're at it, fix a small typo. Change "the git internal" to "the
Git internals".

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>