fix img extension case

Author: lowlydba

docs/sp_estindex.md

@@ -58,7 +58,7 @@ For `[Sales].[Invoices]` in WorldWideImporters:
 *Note: There is no missing index match in this example,
 so the penultimate result set is empty.*
 
-![sp_estindex output](assets/sp_estindex_output.PNG)
+![sp_estindex output](assets/sp_estindex_output.png)
 
 ## Contributing
 

docs/sp_helpme.md

@@ -45,7 +45,7 @@ EXEC sp_helpme 'Sales.Invoices';
 
 For `[Sales].[Invoices]` in WideWorldImporters:
 
-![sp_helpme output](assets/sp_helpme_output.png)
+![sp_helpme output](assets/sp_helpme_output.PNG)
 
 ## Contributing