scalar: fix command documentation section header

scalar: fix command documentation section header

Rename the last section header in 'contrib/scalar/scalar.txt' from "Scalar"
to "GIT". The linting rules of the 'documentation' CI build enforce the
existence of a "GIT" section in command documentation. Although 'scalar.txt'
is not yet checked, it will be in a future patch.

Here, changing the header name is more appropriate than making a
Scalar-specific exception to the linting rule. The existing "Scalar" section
contains only a link back to the main Git documentation, essentially the
same as the "GIT" section in builtin documentation. Changing the section
name further clarifies the Scalar-Git association and maintains consistency
with the rest of Git.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>