Update docs

Author: Flo0807

guides/fields/custom-fields.md

@@ -33,7 +33,7 @@ def render_form(assigns) do
 <div>
     <Layout.field_container>
     <:label>
-        <Layout.input_label text={@field_options[:label]} />
+        <Layout.input_label for={@form[@name]} text={@field_options[:label]} />
     </:label>
     <BackpexForm.input
         type="text"

guides/fields/readonly.md

@@ -58,7 +58,7 @@ def render_form(assigns) do
 <div>
   <Layout.field_container>
     <:label>
-        <Layout.input_label text={@field[:label]} />
+        <Layout.input_label for={@form[@name]} text={@field[:label]} />
     </:label>
     <BackpexForm.input
       type="text"