Fix typo reported in SF bug #427783.

Doc/ref/ref4.tex

@@ -80,7 +80,7 @@ searched in the built-in namespace (which is actually the global
 namespace of the module
 \module{__builtin__}\refbimodindex{__builtin__}).  The built-in
 namespace associated with the execution of a code block is actually
-found by looking up the name \code{__builtins__} is its global
+found by looking up the name \code{__builtins__} in its global
 namespace; this should be a dictionary or a module (in the latter case
 its dictionary is used).  Normally, the \code{__builtins__} namespace
 is the dictionary of the built-in module \module{__builtin__} (note: