#15742: clarify sqlite parameter substitution example.

Suggestion and patch by Mike Hoy.
diff --git a/Doc/library/sqlite3.rst b/Doc/library/sqlite3.rst
index babbfa5..950da95 100644
--- a/Doc/library/sqlite3.rst
+++ b/Doc/library/sqlite3.rst
@@ -60,7 +60,7 @@
    c.execute("select * from stocks where symbol = '%s'" % symbol)
 
    # Do this instead
-   t = (symbol,)
+   t = ('IBM',)
    c.execute('select * from stocks where symbol=?', t)
 
    # Larger example