git-var doc: fix usage of $ENV_VAR vs ENV_VAR
When refering to environment variables in the documentation, use the
ENV_VARIABLE format instead of $ENV_VARIABLE. The latter is used in the
documentation to refer to the actual value of the variable, not the name
of the variable.
Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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