Update README.md

Author: abhinavroy1999

README.md

@@ -4,7 +4,7 @@ The master directory contains two separate directories: -
 1. CahnHilliard2D: -
 This directory contains source code which can perform morphological evolution 
 in a system, governed by the Cahn-Hilliard equation for conserved order parameter. 
-The phenomena which it describes is the well known Spinodal Decomposition in system 
+The phenomenon which it describes is the well known Spinodal Decomposition in system 
 consisting of two phases.
 
 2. AllenCahn2D: -