Fix invalid value set in write_manifest_pid

[lukaszsamson]

Without fixing it

elixir-ls/apps/language_server/lib/language_server/dialyzer.ex

Line 128 in 5b2d236

if state.write_manifest_pid, do: Process.exit(state.write_manifest_pid, :kill)

will crash with ArgumentError and

elixir-ls/apps/language_server/lib/language_server/dialyzer.ex

Line 192 in 5b2d236

if is_pid(write_manifest_pid), do: Process.exit(write_manifest_pid, :cancelled)

will have no effect, which was actually good as exiting a linked process will bring down the caller

[axelson]

❤️

Just making a small wording change to indicate that the write manifest may not be cancelled.