Update the version number preparatory to 5.0.1-alpha

The “prerelease” string will be changed to “alpha” when 5.0.1 is
released.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "tiddlywiki",
   "preferGlobal": "true",
-  "version": "5.0.0-alpha.18-prerelease",
+  "version": "5.0.1-prerelease",
   "author": "Jeremy Ruston <jeremy@jermolene.com>",
   "description": "a non-linear personal web notebook",
   "contributors": [