Fixing path to Configurations.md

[davidalber]

Issue #2419 changed the directory in which configuration_snippet_tests runs, which broke the test. Since the test is currently annotated with #[ignore], the break wasn't caught before merge.

Currently, running cargo test -- --ignored on master currently yields the following.

---- configuration_snippet_tests stdout ----
        thread 'configuration_snippet_tests' panicked at 'Couldn't read file Configurations.md: Os { code: 2, kind: NotFound, message: "No such file or directory" }', libcore/result.rs:945:5

Running cargo test -- --ignored on the PR branch yields the following.

.
.
.
---- configuration_snippet_tests stdout ----
        Ran 106 configurations tests.
thread 'configuration_snippet_tests' panicked at 'assertion failed: `(left == right)`
  left: `4`,
 right: `0`: 4 configurations tests failed', rustfmt-core/tests/lib.rs:817:5

[topecongiro]

Oops, thank you for catching this!