Fix typo

Author: nicholasbishop

BUILDING

@@ -27,7 +27,7 @@ Variables you should set to customize the build:
 
 Variables you could set to customize the build:
 - ENABLE_SHIM_CERT
-  if this variable is defined one the make command line, shim will
+  if this variable is defined on the make command line, shim will
   generate keys during the build and sign MokManager and fallback with
   them, and the signed version will be what gets installed with the
   install targets