fix minor typo in storage.md docs (#772)

content/docs/operating/storage.md

@@ -165,7 +165,7 @@ scrape (see `sample_limit` in the
 [scrape config](/docs/operating/configuration/#scrape_config)).
 If the number of active time series exceeds the number of memory chunks the
 Prometheus server can afford, the server will quickly throttle ingestion as
-described above. The only way out if this is to give Prometheus more RAM or
+described above. The only way out of this is to give Prometheus more RAM or
 reduce the number of time series to ingest.
 
 In fact, you want many more memory chunks than you have series in