builder.md: Correct copy/paste error

Signed-off-by: Daniel Cassidy <mail@danielcassidy.me.uk>
(cherry picked from commit e9ada56a888a042476f370d2de45b0633f7f85f6)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 66f8d279c1dc7f0c18b05c6dc8b39a79b5c18fdf
Component: cli

Author: djcsdy

components/cli/docs/reference/builder.md

@@ -1720,8 +1720,8 @@ The following `ARG` variables are set automatically:
 * `TARGETVARIANT` - variant component of TARGETPLATFORM
 * `BUILDPLATFORM` - platform of the node performing the build.
 * `BUILDOS` - OS component of BUILDPLATFORM
-* `BUILDARCH` - OS component of BUILDPLATFORM
-* `BUILDVARIANT` - OS component of BUILDPLATFORM
+* `BUILDARCH` - architecture component of BUILDPLATFORM
+* `BUILDVARIANT` - variant component of BUILDPLATFORM
 
 These arguments are defined in the global scope so are not automatically
 available inside build stages or for your `RUN` commands. To expose one of