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

jeremyhylton · jeremyhylton · commit 1336e9905417 · 2001-11-14T21:38:13.000Z
diff --git a/Doc/ref/refa1.tex b/Doc/ref/refa1.tex
@@ -172,7 +172,7 @@ \subsection{Definitions and rules \label{definitions}}
 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.
 
