updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -55,7 +55,7 @@
 
 ## Blockchain
   * [Diophantine Equation](blockchain/diophantine_equation.py)
-  * [Pow Algorithm](blockchain/pow_algorithm.py)
+  * [Proof Of Work](blockchain/proof_of_work.py)
 
 ## Boolean Algebra
   * [And Gate](boolean_algebra/and_gate.py)