build: fix LINT_MD_NEWER assignment

Indentation with a tab breaks the functionality, resulting in linting
all .md files when any one is changed. For consistency with the rest of
the Makefile and to restore functionality, remove indentation.

Refs: https://github.com/nodejs/node/pull/32614#issuecomment-610670779

PR-URL: https://github.com/nodejs/node/pull/32712


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>