Fix a small typo in the documentation

Author: ealter

doc/requirements-and-limitations.md

@@ -45,7 +45,7 @@ The `SUPER` privilege is required for `STOP SLAVE`, `START SLAVE` operations. Th
 
 - Master-master setup is only supported in active-passive setup. Active-active (where table is being written to on both masters concurrently) is unsupported. It may be supported in the future.
 
-- If you have en `enum` field as part of your migration key (typically the `PRIMARY KEY`), migration performance will be degraded and potentially bad. [Read more](https://github.com/github/gh-ost/pull/277#issuecomment-254811520)
+- If you have an `enum` field as part of your migration key (typically the `PRIMARY KEY`), migration performance will be degraded and potentially bad. [Read more](https://github.com/github/gh-ost/pull/277#issuecomment-254811520)
 
 - Migrating a `FEDERATED` table is unsupported and is irrelevant to the problem `gh-ost` tackles.