Update readme.md

fix getValue(...) to $db->getValue(...) in Select Query section of the readme file

Author: hedii

readme.md

@@ -142,7 +142,7 @@ echo "total ".$stats['cnt']. "users found";
 or select one column or function result
 
 ```php
-$count = getValue ("users", "count(*)");
+$count = $db->getValue ("users", "count(*)");
 echo "{$count} users found";
 ```