Cargo.toml: replace deprecated option with new version

I was getting this log message during run: ``` pro_release_commit_message is deprecated, use post-release-commit-message instead ```
coreos · Jan 8, 2020 · 0707602 · 0707602
1 parent 42acdb3
commit 0707602
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/Cargo.toml b/Cargo.toml
@@ -13,7 +13,7 @@ sign-commit = true
 disable-push = true
 disable-publish = true
 pre-release-commit-message = "cargo: coreos-installer release {{version}}"
-pro-release-commit-message = "cargo: development version bump"
+post-release-commit-message = "cargo: development version bump"
 tag-message = "coreos-installer v{{version}}"
 
 [[bin]]