Issue #23070: Fix a comment in the tutorial.

"Python" has 6 characters, not 7.

Reported by Ross Burnett.
diff --git a/Doc/tutorial/introduction.rst b/Doc/tutorial/introduction.rst
index d30b194..4164d49 100644
--- a/Doc/tutorial/introduction.rst
+++ b/Doc/tutorial/introduction.rst
@@ -309,7 +309,7 @@
 
 Attempting to use a index that is too large will result in an error::
 
-   >>> word[42]  # the word only has 7 characters
+   >>> word[42]  # the word only has 6 characters
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    IndexError: string index out of range