Fix name of the Slack template input

Using a space to separate the two words look like it would work better than a
hyphen ('-') as you can't have a hyphen in an environment variable, which is
necessary for testing this locally.

Spaces are however explicitly changed to underscores in the @actions/core
library so it's clear how that would work, and it's prettier than using an
underscore through out.

README.md

@@ -42,7 +42,7 @@ Three different output formats are supplied, plain text, Markdown and for Slack.
     # - until.short
     #
     # Default: *Changes from {since.short} to {until.short}*
-    slacktemplate: ''
+    slack template: ''
 ```
 
 

action.yml

@@ -15,7 +15,7 @@ inputs:
   chronological:
     description: Output changes in chronological order
     default: true
-  slack-template:
+  slack template:
     description: |
       Message template to be used for line above the Slack output.
 

src/main.ts

@@ -58,7 +58,7 @@ async function run(): Promise<void> {
 
   const since = core.getInput('since') || (await getMostRecentRelease(octokit)) || (await getMostRecentTag(octokit))
   let until = core.getInput('until', {required: true})
-  const slackTemplate = core.getInput('slacktemplate')
+  const slackTemplate = core.getInput('slack template')
 
   if (!since) {
     core.setFailed("`since` was not set and a reasonable default couldn't be established")