docs: update entity-manager-api.md (typeorm#9956)

typo:
const timber = await manager.findOne(User, { firstName: "Timber" })   →   const timber = await manager.findOneBy(User, { firstName: "Timber" })

Co-authored-by: Jo YoHan <37216082+oxyrinchus@users.noreply.github.com>

docs/entity-manager-api.md

@@ -252,7 +252,7 @@ const timber = await manager.findOne(User, {
 -   `findOneBy` - Finds the first entity that matches given `FindOptionsWhere`.
 
 ```typescript
-const timber = await manager.findOne(User, { firstName: "Timber" })
+const timber = await manager.findOneBy(User, { firstName: "Timber" })
 ```
 
 -   `findOneOrFail` - Finds the first entity that matches some id or find options.