Update Disable Cache Expiration Docs

The documentation says to set the `ttl` to `null` should disable the cache. The TS types do not allow it to be null, and I found that actually setting it to null does not disable the cache expiration. It looks like setting the ttl to `0` is actually the way to disable the expiration, at least for the memory store and some other stores.

jaredwray/cacheable#107

jaredwray/cacheable#147

Author: ncausey

content/techniques/caching.md

@@ -58,10 +58,10 @@ You can manually specify a TTL (expiration time) for this specific key, as follo
 await this.cacheManager.set('key', 'value', { ttl: 1000 });
 ```
 
-To disable expiration of the cache, set the `ttl` configuration property to `null`:
+To disable expiration of the cache, set the `ttl` configuration property to `0`:
 
 ```typescript
-await this.cacheManager.set('key', 'value', { ttl: null });
+await this.cacheManager.set('key', 'value', { ttl: 0 });
 ```
 
 To remove an item from the cache, use the `del` method: