Fix misplaced parenthesis (#3401)

The implementation given in the docs differs from the implementation actually used. The implementation currently shown in the docs raises an AttributeError when used. It appears that the docs were originally created with an extra stray close paren, and the wrong one was deleted when that was fixed.

docs/types/scalars.md

@@ -113,7 +113,7 @@ import strawberry
 Base64 = strawberry.scalar(
     NewType("Base64", bytes),
     serialize=lambda v: base64.b64encode(v).decode("utf-8"),
-    parse_value=lambda v: base64.b64decode(v).encode("utf-8"),
+    parse_value=lambda v: base64.b64decode(v.encode("utf-8")),
 )