Update INSTALL

Corrected the instructions for modifying the 
configuration files to refer to the new file 
layout (localOverrides.conf vs. old global.conf).

Author: djun-kim

INSTALL

@@ -28,7 +28,7 @@ is refered to as the "problem library"
 2. Tell WeBWorK where on your disk you put the problem library
 --------------------------------------------------------------
 
-In the file $WEBWORK_ROOT/conf/global.conf, set the following
+In the file $WEBWORK_ROOT/conf/localOverrides.conf, set the following
 values.  The first one is where you put the problem library files
 in step 1.