Fix writeable typo in an expire test name (#4499)

One test name spells it `writeable`. Outside `deps/` that is the only
occurrence left in the tree, everywhere else is `writable`.

Current releases of `typos` flag it, and the Spellcheck job installs
`typos` through `taiki-e/install-action` without pinning a version, so
the job can turn red without anyone touching this repo. With this word
changed, `typos --config=./.config/typos.toml` is clean across the tree.

It is a test name, so nothing changes behaviour, and nothing else refers
to the old one.

Signed-off-by: melancholictheory <selimvhorst@gmail.com>

Author: melancholictheory

tests/unit/expire.tcl

@@ -610,7 +610,7 @@ start_server {tags {"expire"}} {
             }
         }
 
-        test {expired key which is created in writeable replicas should be deleted by active expiry} {
+        test {expired key which is created in writable replicas should be deleted by active expiry} {
             $primary flushall
             $replica config set replica-read-only no
             foreach {yes_or_no} {yes no} {