Changed proptypes in PlaceholderCommon

[Jeffrey-Zutt]

Description

Changed proptypes of missingComponentComponent and errorComponent to PropTypes.func in
packages/sitecore-jss-react/src/components/PlaceholderCommon.tsx

Motivation

Whenever the following JSX is used:

<Placeholder 
   missingComponentComponent={MissingComponentComponent} 
   errorComponent={ErrorComponent}  {...placeholderProps} 
/>

The following warning will occur:

Warning: Failed prop type: Invalid prop `missingComponentComponent` of type `function` supplied to `PlaceholderComponent`, expected `object`.

This PR will stop that warning from showing

How Has This Been Tested?

I've changed the proptypes and made sure the warning did not show up in my browser-console.

Browser: Chrome Version 73.0.3683.86 (Official Build) (64-bit)
OS: OSX

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the Contributing guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.

[aweber1]

Thanks for the contribution!