fixing env var in build help (#1130)

libexec/cli/build.info

@@ -90,7 +90,7 @@ DEFFILE SECTION EXAMPLES:
     %setup
         echo "This is a scriptlet that will be executed on the host, as root, after"
         echo "the container has been bootstrapped. To install things into the container"
-        echo "reference the file system location with \$SINGULARITY_BUILDROOT"
+        echo "reference the file system location with \$SINGULARITY_ROOTFS"
 
     %post
         echo "This scriptlet section will be executed from within the container after"