Fix typo reported by François Pinard.

Doc/ref/refa1.tex

@@ -174,7 +174,7 @@ containing the use.
 
 A \dfn{block} is a piece of Python program text that is executed as
 a unit.  The following are blocks: a module, a function body, and a
-class defintion.
+class definition.
 
 A \dfn{scope} defines the visibility of a name within a block.  If a
 local variable is defined in a block, it's scope includes that block.