Fix typo

Closes GH-868.

Reviewed-by: Remco Haszing <[email protected]>
Reviewed-by: Titus Wormer <[email protected]>

readme.md

@@ -678,7 +678,7 @@ const markdown = `
 # This is perfect!
 `
 
-// Pass the value as an expresion as an only child:
+// Pass the value as an expression as an only child:
 const result = <Markdown>{markdown}</Markdown>
 ```