use single quotes to wrap sheet title

This changes is to clarify that the sheet title is used to reference the sheet and that the title must be single quoted.

Author: jomisacu

docs/topics/recipes.md

@@ -1104,7 +1104,7 @@ formula is allowed to be maximum 255 characters (not bytes). This sets a
 limit on how many items you can have in the string "Item A,Item B,Item
 C". Therefore it is normally a better idea to type the item values
 directly in some cell range, say A1:A3, and instead use, say,
-`$validation->setFormula1('Sheet!$A$1:$A$3')`. Another benefit is that
+`$validation->setFormula1('\'Sheet title\'!$A$1:$A$3')`. Another benefit is that
 the item values themselves can contain the comma `,` character itself.
 
 If you need data validation on multiple cells, one can clone the