docs: fix indent of builtins and nix.conf descriptions in manual
The manual for the builtins and nix.conf currently has inconsistent
indentation, which causes some of the descriptions to end up being
partially treatedas code blocks in markdown (and thus the manual).
This was simply caused by the template string for the docs having
too much indentation before the description is inserted, so this fixes
that 16-bytes mistake.
Change-Id: Ia264e3b1abb20430109029d07a2d2b0a1a726bd4
(cherry picked from commit 8294cd534b)
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