Fix grammar error in provenance min mode description

Fixes subject-verb agreement ("is" → "are") and adds missing
preposition "in" before `mode=min`.

Closes #24664

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dvdksn

content/manuals/build/metadata/attestations/slsa-provenance.md

@@ -59,8 +59,8 @@ such as:
 - Build platform
 - Reproducibility
 
-Values of build arguments, the identities of secrets, and rich layer metadata is
-not included `mode=min`. The `min`-level provenance is safe to use for all
+Values of build arguments, the identities of secrets, and rich layer metadata are
+not included in `mode=min`. The `min`-level provenance is safe to use for all
 builds, as it doesn't leak information from any part of the build environment.
 
 The following JSON example shows the information included in a provenance