Update the builtins documentation.

	* doc/builtins.texi: Sync with tc-assignments.

doc/builtins.texi

@@ -98,7 +98,7 @@ string equality.
 Return a string composed of the characters of @var{string} starting at
 the @var{first} character (0 being the origin), and composed of
 @var{length} characters (i.e., up to and including the character
-@var{first} + @var{length}).
+@var{first} + @var{length} - 1).
 
 Let @var{size} be the size of the @var{string}, the following assertions
 must hold: