docs: fix input text color to match theme in custom form guide (#21750)

Splaktar · web-flow · commit e773f9078bec · 2021-02-05T17:57:40.000+08:00
- without this, it's black on dark grey
diff --git a/guides/creating-a-custom-form-field-control.md b/guides/creating-a-custom-form-field-control.md
@@ -42,6 +42,7 @@ class MyTel {
       outline: none;
       font: inherit;
       text-align: center;
+      color: currentColor;
     }
   `],
 })
