Update README.md

Updated instructions to fix any grammar errors.

subjects/isalpha/README.md

@@ -2,7 +2,7 @@
 
 ### Instructions
 
-Write a function that returns `true` if the `string` passed in parameter only contains alphanumerical characters or is empty, and returns `false` otherwise.
+Write a function that returns `true` if the `string` passed as the parameter contains alphanumerical characters; returns `false` otherwise.
 
 ### Expected function