Update README.md

Author: byte271

README.md

@@ -365,7 +365,7 @@ the vulnerability disclosure policy.
 
 ## Container Format Specification
 
-See [`SPEC.md`](SPEC.md)
+See [`spec.md`](spec.md)
 
 ## Contributing
 
@@ -385,3 +385,4 @@ and share implementations freely, provided you attribute the original
 specification.
 
 
+