fixed a typo

Author: chmerdon

examples/Example312_PeriodicBoundary3D.jl

@@ -5,7 +5,7 @@
 
 This is a simple demonstration and validation of the generic periodic boundary operator.
 
-We construct an unstructured periodic 2D grid and solve a simple linear elastic problem
+We construct an unstructured periodic 3D grid and solve a simple linear elastic problem
 with periodic coupling along the x-axis.
 
 ![](example312.png)