Fixing the build break

Summary: The swift package version 14.0-Snapshot is not released externally. This diff downgrades the dependency to 13.2 which is the latest swift version released externally. For example, see: https://oss.sonatype.org/index.html#nexus-search;gav~com.facebook.swift~swift-generator~~~~kw,versionexpand

Test Plan: built on my devbox

Reviewers: MarkCallaghan

Reviewed By: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D23985