Fred observes that the typo was not the only problem with this sentence.

Doc/ref/refa1.tex

@@ -172,7 +172,7 @@ operations.  Each occurrence of a name in the program text refers to
 the binding of that name established in the innermost function block
 containing the use.
 
-A \dfn{block} is a piece of Python program text that can is executed as
+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.