Bump proc_macro2 to support building on nightly rust

[joshuataylor]

Nightly rust seems to need a bump of proc_macro2.

Without this, the following error occurs:

   Compiling proc-macro2 v1.0.52
   Compiling fd-find v8.7.0 (/home/josh/dev/fd)
error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/josh/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.52/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0635`.
error: could not compile `proc-macro2` (lib) due to previous error

I just ran cargo update proc-macro2, which fixes the issue.