fix typo and join two paragraphs

Doc/ref/ref4.tex

@@ -80,8 +80,7 @@ If a name binding operation occurs anywhere within a code block, all
 uses of the name within the block are treated as references to the
 current block.  This can lead to errors when a name is used within a
 block before it is bound.
-
-The previous rule is a subtle.  Python lacks declarations and allows
+This rule is subtle.  Python lacks declarations and allows
 name binding operations to occur anywhere within a code block.  The
 local variables of a code block can be determined by scanning the
 entire text of the block for name binding operations.