docs: properly annotate forms disabled field (#486)

See #467 (comment)

selfservice/form/fields.go

@@ -35,7 +35,7 @@ type Field struct {
  // Pattern is the equivalent of `<input pattern="{{.Pattern}}">`
  Pattern string `json:"pattern,omitempty"`
 
- // Disabled is the equivalent of `<input disabled="{{.Disabled}}">`
+ // Disabled is the equivalent of `<input {{if .Disabled}}disabled{{end}}">`
  Disabled bool `json:"disabled,omitempty"`
 
  // Required is the equivalent of `<input required="{{.Required}}">`