Improve wording of Local Variables and eval section

As pointed out by nobu, "defined" should be used instead of "assigned".

doc/syntax/assignment.rdoc

@@ -111,14 +111,14 @@ method.
 
 == Local Variables and eval
 
-Using +eval+ to evaluate Ruby code will allow access to local variables in
-the same scope, even if the local variables are not assigned until after the
-call to +eval+.  However, local variables assigned inside the call to +eval+
+Using +eval+ to evaluate Ruby code will allow access to local variables defined
+in the same scope, even if the local variables are not defined until after the
+call to +eval+.  However, local variables defined inside the call to +eval+
 will not be reflected in the surrounding scope.  Inside the call to +eval+,
-local variables in the scope and local variables assigned inside the call to
-+eval+ will be accessible.  However, you will not be able to access local
-variables assigned in previous or subsequent calls to +eval+ in the same
-scope. Consider each +eval+ call a separate nested scope. Example:
+local variables defined in the surrounding scope and local variables defined
+inside the call to +eval+ will be accessible.  However, you will not be able
+to access local variables defined in previous or subsequent calls to +eval+ in
+the same scope. Consider each +eval+ call a separate nested scope. Example:
 
   def m
     eval "bar = 1"