fix: Updated RichTextInputBlockElement InitialValue data type (slack-…

…go#1320)

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##### Should this be an issue instead
- [x] is it a convenience method? (no new functionality, streamlines
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- [ ] is it application specific (caching, retry logic, rate limiting,
etc)
- [ ] is it performance related.

Fix for [issue slack-go#1276](slack-go#1276)
Updated the datatype of RichTextInputBlockElement InitialValue from
string to *RichTextBlock

block_element.go

@@ -527,7 +527,7 @@ type RichTextInputBlockElement struct {
 	Type                 MessageElementType    `json:"type"`
 	ActionID             string                `json:"action_id,omitempty"`
 	Placeholder          *TextBlockObject      `json:"placeholder,omitempty"`
-	InitialValue         string                `json:"initial_value,omitempty"`
+	InitialValue         *RichTextBlock        `json:"initial_value,omitempty"`
 	DispatchActionConfig *DispatchActionConfig `json:"dispatch_action_config,omitempty"`
 	FocusOnLoad          bool                  `json:"focus_on_load,omitempty"`
 }