Fix Pettern Matching typo

Author: iantabolt

docs/sidebar.yml

@@ -63,7 +63,7 @@ sidebar:
               url: docs/reference/changed/implicit-conversions.html
             - title: Vararg Patterns
               url: docs/reference/changed/vararg-patterns.html
-            - title: Pettern matching
+            - title: Pattern matching
               url: docs/reference/changed/pattern-matching.html
         - title: Dropped Features
           subsection: