Fix trivial difference between full and non-full expression parse errors

[dtolnay]

This silly difference causes serde_derive's UI test suite to fail depending on whether syn's "full" feature is turned on.

syn/src/expr.rs

Line 1666 in f79ba15

Err(input.error("expected expression"))

syn/src/expr.rs

Line 1692 in f79ba15

Err(input.error("expected an expression"))

test tests/ui/malformed/cut_off.rs ... mismatch

EXPECTED:
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
error: unexpected end of input, expected an expression
 --> tests/ui/malformed/cut_off.rs:4:17
  |
4 | #[serde(rename =)]
  |                 ^
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈

ACTUAL OUTPUT:
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
error: unexpected end of input, expected expression
 --> tests/ui/malformed/cut_off.rs:4:17
  |
4 | #[serde(rename =)]
  |                 ^
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
note: If the actual output is the correct output you can bless it by rerunning
      your test with the environment variable TRYBUILD=overwrite