Update defs.markdown

Similar typo correction as in sourcemeta#283 

Signed-off-by: Danny van der Sluijs <[email protected]>

content/2020-12/core/defs.markdown

@@ -112,7 +112,7 @@ The `$defs` keyword provides a standardized way to define reusable subschemas wi
 }
 {{</instance-pass>}}
 
-{{<instance-fail `'author' proeprty is required`>}}
+{{<instance-fail `'author' property is required`>}}
 {
   "title": "Fundamental Chemistry"
 }