Fix error message typo in validateStrings function

Remove extra single quote in error message template that was causing
malformed output like `"'namespace" must be a string` instead of the
correct `"namespace" must be a string`.

Author: jdalton

src/validate.ts

@@ -158,7 +158,7 @@ function validateStrings(
     return true
   }
   if (throws) {
-    throw new PurlError(`"'${name}" must be a string`)
+    throw new PurlError(`"${name}" must be a string`)
   }
   return false
 }