[basic.pre] Clarify definition of variable

Reorder the positive and negative terms to avoid confusion with
the binding of "other than".  While grammatically the current
wording is precise and correct, I have run into several folks
misreading this and wondering why references to objects are not
variables.

Author: AlisdairM

source/basic.tex

@@ -113,8 +113,8 @@
 \pnum
 A \defn{variable} is introduced by the
 declaration of
-a reference other than a non-static data member or of
-an object.
+an object or of
+a reference other than a non-static data member.
 
 \pnum
 An \defn{entity} is a