Update mermaid.mmd

The alt-parallelogram didn't use the backslash for the alternate direction so it didn't render it correctly

Author: binarynomad

src/examples/2_shapes/syntax/mermaid.mmd

@@ -15,7 +15,7 @@ flowchart TD
     Database --- Asymetric --- Hexagon
 
     Parallelogram[/Parallelogram/]
-    AltParallelogram[/"Parallelogram (alt)"/]
+    AltParallelogram[\"Parallelogram (alt)"\]
     Parallelogram --- AltParallelogram
 
     Trapezoid[/Trapezoid\] --- AltTrapezoid[\"Trapezoid (alt)"/]