tweak validation regex for InstanceType param

the 1-4 values must be comma separated with no whitespace. Previously
the validation accepted "m4.large, m5.large", now it does not.

Author: yob

templates/aws-stack.yml

@@ -208,7 +208,7 @@ Parameters:
     Type: String
     Default: t3.large
     MinLength: 1
-    AllowedPattern: "^[^,]+(,[^,]*){0,3}$"
+    AllowedPattern: "^[\\w\\.]+(,[\\w\\.]*){0,3}$"
     ConstraintDescription: "must contain 1-4 instance types separated by commas. No space before/after the comma."
 
   MaxSize: