builder.md: Correct copy/paste error
@@ -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
