docs: fix a capitalization problem in messagePort tutorial (#39130)

fix a capitalization problem

For a rookie, this little mistake took a long time to find out.

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: ZWJKFLC <[email protected]>

docs/tutorial/message-ports.md

@@ -150,7 +150,7 @@ renderer.
 
 ```js title='renderer.js (Renderer Process)' @ts-nocheck
 // elsewhere in your code to send a message to the other renderers message handler
-window.electronMessagePort.postmessage('ping')
+window.electronMessagePort.postMessage('ping')
 ```
 
 ### Worker process