Fix elided_named_lifetimes

[yubiuser]

I was compiling librespot with the nightly toolchain an noticed this warning

#10 106.1    Compiling librespot-connect v0.5.0-dev (/librespot/connect)
#10 106.2 warning: elided lifetime has a name
#10 106.2     --> connect/src/spirc.rs:1537:73
#10 106.2      |
#10 106.2 1536 | impl<'a> CommandSender<'a> {
#10 106.2      |      -- lifetime `'a` declared here
#10 106.2 1537 |     fn new(spirc: &'a mut SpircTask, cmd: MessageType) -> CommandSender<'_> {
#10 106.2      |                                                                         ^^ this elided lifetime gets resolved as `'a`
#10 106.2      |
#10 106.2      = note: `#[warn(elided_named_lifetimes)]` on by default
#10 106.2 
#10 106.2 warning: elided lifetime has a name
#10 106.2     --> connect/src/spirc.rs:1552:65
#10 106.2      |
#10 106.2 1536 | impl<'a> CommandSender<'a> {
#10 106.2      |      -- lifetime `'a` declared here
#10 106.2 ...
#10 106.2 1552 |     fn recipient(mut self, recipient: &'a str) -> CommandSender<'_> {
#10 106.2      |                                                                 ^^ this elided lifetime gets resolved as `'a`
#10 106.2 

Background: rust-lang/rust#129207

This PR fixes the warning.