Docs: Fix a typo in the 'Non-ASCII characters in names' section (python#148043)

Author: StanFromIreland

Doc/reference/lexical_analysis.rst

@@ -560,7 +560,7 @@ start with a character in the "letter-like" set ``xid_start``,
 and the remaining characters must be in the "letter- and digit-like" set
 ``xid_continue``.
 
-These sets based on the *XID_Start* and *XID_Continue* sets as defined by the
+These sets are based on the *XID_Start* and *XID_Continue* sets as defined by the
 Unicode standard annex `UAX-31`_.
 Python's ``xid_start`` additionally includes the underscore (``_``).
 Note that Python does not necessarily conform to `UAX-31`_.