[third-party/rust] Rename syn version 2 to syn2

Summary: In preparation for syn v3 to be `third-party/rust:syn`

Test Plan: `xbgs third-party/rust:syn -l | sed 's,^fbsource/,,' | rg -v -e '^third-party/rust/' -e '^users/' -e '^whatsapp/' -e '^fbcode/scripts/' -e '^third-party/cloud-hypervisor/' -e '^third-party/mtia-simt-dpa/' -e '^third-party/nativelink/' | xargs arc rust-check`

Reviewed By: boujeepossum, edward-shen

Differential Revision: D114630378

fbshipit-source-id: 713af16b3d1e57b1d976a5fd3325d36ff04824d9

Author: David Tolnay

antlir/antlir2/antlir2_facts/BUCK

@@ -51,6 +51,6 @@ rust_library(
         "proc-macro-error2",
         "proc-macro2",
         "quote",
-        "syn",
+        "syn2",
     ],
 )