correct midpoint example

source/Scrolls/README.md

@@ -55,7 +55,7 @@ theory Midpoint =
 
         midpoint
             : point ❘
-            = ⟨0.5 ⋅ (P_x + Q_x), 0.5 ⋅ (P_x + Q_x), 0.5 ⋅ (P_x + Q_x)⟩ ❘
+            = ⟨0.5 ⋅ (P _x + Q _x), 0.5 ⋅ (P _y + Q _y), 0.5 ⋅ (P _z + Q _z)⟩ ❘
             meta ?MetaAnnotations?label s"Mid[${lverb P Q}]" ❘
             meta ?MetaAnnotations?description s"The midpoint between points ${lverb P} and ${lverb Q}."
         ❙