[1.5.x] Added missing preposition in documentation

Backport of 2ec54e7fbc from master

docs/howto/custom-model-fields.txt

@@ -31,7 +31,7 @@ Our example object
 Creating custom fields requires a bit of attention to detail. To make things
 easier to follow, we'll use a consistent example throughout this document:
 wrapping a Python object representing the deal of cards in a hand of Bridge_.
-Don't worry, you don't have know how to play Bridge to follow this example.
+Don't worry, you don't have to know how to play Bridge to follow this example.
 You only need to know that 52 cards are dealt out equally to four players, who
 are traditionally called *north*, *east*, *south* and *west*.  Our class looks
 something like this::