[Doc] Fix typo in ReferenceManyInput documentation

Author: fzaninotto

docs/ReferenceManyInput.md

@@ -209,7 +209,7 @@ By default, react-admin restricts the possible values to 25 and displays no pagi
 
 `<SimpleFormIterator>` provides controls to reorder the items in the list. If the related records have a numeric rank field, you can enable the reordering feature by setting the `rankSource` prop.
 
-for example, if the variants have a `rank` field, you can set the `rankSource` prop like this:
+For example, if the variants have a `rank` field, you can set the `rankSource` prop like this:
 
 ```jsx
 <ReferenceManyInput