oops. Fix missing end

Author: ablaom

docs/src/maths/propagators.md

@@ -191,9 +191,10 @@ end
 function ChainRulesCore.rrule(m::Multiplier, y)
     product = m(y)
     function pullback(Δproduct) 
-	    ∂self = Tangent{typeof(m)}(; x = Δproduct * y')
-	    ∂y = m.x' * Δproduct
-	    return ∂self, ∂y
+        ∂self = Tangent{typeof(m)}(; x = Δproduct * y')
+        ∂y = m.x' * Δproduct
+        return ∂self, ∂y
+    end 
     return product, pullback
 end
 ```