bpo-32735: Fix typo in f-strings datetime format specifier example (GH-5464)

The f-string example for using datetime format specifier does not match the given output.
Changed the format from %b to %B so it matches the output of "January".
diff --git a/Doc/reference/lexical_analysis.rst b/Doc/reference/lexical_analysis.rst
index 95b0f53..c6d96ca 100644
--- a/Doc/reference/lexical_analysis.rst
+++ b/Doc/reference/lexical_analysis.rst
@@ -676,7 +676,7 @@
    >>> f"result: {value:{width}.{precision}}"  # nested fields
    'result:      12.35'
    >>> today = datetime(year=2017, month=1, day=27)
-   >>> f"{today:%b %d, %Y}"  # using date format specifier
+   >>> f"{today:%B %d, %Y}"  # using date format specifier
    'January 27, 2017'
    >>> number = 1024
    >>> f"{number:#0x}"  # using integer format specifier