Allow `'s` (#861)

21 jobs for master in 7 minutes and 4 seconds (queued for 2 seconds)
Name Stage Failure
failed
spellcheck Workspace
    --> /builds/parity/ink/crates/lang/ir/src/ir/trait_def.rs:360
|
[2021-07-19T12:26:27Z INFO cargo_spellcheck::action] ❌ /builds/parity/ink/crates/lang/ir/src/ir/trait_def.rs : 1
360 | - unknown token sequences (`Verbatim`'s)
| ^
| Possible spelling mistake found.

Cleaning up file based variables
ERROR: Job failed: exit code 1