Fix typo in example code block for the Filesource provider (elastic#3336)

This PR fixes a typo in the code block example for configuring a
Filesource provider on a standalone Elastic Agent.

Author: vishaangelova

reference/fleet/filesource-provider.md

@@ -7,7 +7,7 @@ This allows information from the filesystem to be used as variables in the {{age
 For example, the following configuration watches for changes to `file1`:
 
 ```yaml
-provides:
+providers:
   filesource:
     sources:
       file1:
@@ -18,4 +18,4 @@ inputs:
    type: filestream
    paths:
      - ${filesource.file1}
-```
+```