Revert "Give spheres positive radii in examples"

This reverts commit 4728959ae0, which
actually gave the spheres negative radii! I got confused by the sign
convention differences between the notes and the engine.

engine-proto/gram-test/sphere-in-tetrahedron.jl

@@ -28,7 +28,7 @@ for j in 1:6
       push!(values, -1/BigFloat(3))
       filled = true
     else
-      push!(values, 1)
+      push!(values, -1)
       filled = true
     end
     if filled
@@ -46,8 +46,8 @@ guess = hcat(
     1  1 -1 -1  0
     1 -1  1 -1  0
     1 -1 -1  1  0
-    0  0  0  0  1.5
-    1  1  1  1  0.5
+    0  0  0  0 -1.5
+    1  1  1  1 -0.5
   ] + 0.2*Engine.rand_on_shell(fill(BigFloat(-1), 5)),
   BigFloat[0, 0, 0, 0, 1]
 )