dicts.bzl fix

Summary:
Fix error message printing. When merging dicts failed because of non-equal, duplicate entries, the failure message would fail to print.

It would error that the format string was invalid in the the failure, not the actual error message

Reviewed By: IanChilds

Differential Revision: D66457797

fbshipit-source-id: 609cf0b41ab0e8037c1fa542951d6a122ff14ade

prelude/utils/dicts.bzl

@@ -7,7 +7,7 @@
 
 load("@prelude//utils:expect.bzl", "expect")
 
-_DEFAULT_FMT = "found different values for key \"{0}\": {} != {}"
+_DEFAULT_FMT = "found different values for key \"{}\": {} != {}"
 
 def update_x(dst: dict[typing.Any, typing.Any], k: typing.Any, v: typing.Any, fmt = _DEFAULT_FMT):
     p = dst.setdefault(k, v)