Bump tokio from 1.19.2 to 1.20.3 in /examples/signaling-server-rust

Bumps [tokio](https://github.com/tokio-rs/tokio) from 1.19.2 to 1.20.3.
- [Release notes](https://github.com/tokio-rs/tokio/releases)
- [Commits](https://github.com/tokio-rs/tokio/compare/tokio-1.19.2...tokio-1.20.3)

---
updated-dependencies:
- dependency-name: tokio
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

examples/signaling-server-rust/Cargo.lock

@@ -223,7 +223,7 @@ checksum = "349d5a591cd28b49e1d1037471617a32ddcda5731b99419008085f72d5a53836"
 
 [[package]]
 name = "libdatachannel_signaling_server_example"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "futures-channel",
  "futures-util",
@@ -286,12 +286,6 @@ dependencies = [
  "libc",
 ]
 
-[[package]]
-name = "once_cell"
-version = "1.12.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7709cef83f0c1f58f666e746a08b21e0085f7440fa6a29cc194d68aac97a4225"
-
 [[package]]
 name = "parking_lot"
 version = "0.12.1"
@@ -492,16 +486,16 @@ checksum = "cda74da7e1a664f795bb1f8a87ec406fb89a02522cf6e50620d016add6dbbf5c"
 
 [[package]]
 name = "tokio"
-version = "1.19.2"
+version = "1.20.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c51a52ed6686dd62c320f9b89299e9dfb46f730c7a48e635c19f21d116cb1439"
+checksum = "664cf6576f16c0ad68184998f3c2d4be9903ede6b291a5b5cfc97d29e0057283"
 dependencies = [
+ "autocfg",
  "bytes",
  "libc",
  "memchr",
  "mio",
  "num_cpus",
- "once_cell",
  "parking_lot",
  "pin-project-lite",
  "signal-hook-registry",