fix: regression from #1123

Since we are building the regex with `new RegExp`, the escapes must be
double-escaped. Instead of `\.` to esape the period, we need `\\.`.
Because that was not done, we were simply checking for *any* character,
instead of a literal period.

Author: tech4him1

packages/netlify-cms-core/src/reducers/collections.js

@@ -39,7 +39,7 @@ const selectors = {
       return `${ collection.get('folder').replace(/\/$/, '') }/${ slug }.${ this.entryExtension(collection) }`;
     },
     entrySlug(collection, path) {
-      return path.split('/').pop().replace(new RegExp(`\.${ escapeRegExp(this.entryExtension(collection)) }$`), '');
+      return path.split('/').pop().replace(new RegExp(`\\.${ escapeRegExp(this.entryExtension(collection)) }$`), '');
     },
     listMethod() {
       return 'entriesByFolder';