minor #18028 [Session] Fix an RST syntax issue (javiereguiluz)

This PR was merged into the 5.4 branch.

Discussion
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[Session] Fix an RST syntax issue

Fixes this issue:

<img width="931" alt="error-1" src="https://user-images.githubusercontent.com/73419/223742820-e638aa9a-6856-4a90-8113-7ac41fce863e.png">

Commits
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c02b87d [Session] Fix an RST syntax issue

Author: javiereguiluz

session.rst

@@ -1129,9 +1129,6 @@ These are parameters that you can configure:
 ``expiry_field`` (default ``expires_at``):
     The name of the field where to store the session lifetime.
 
-.. index::
-    single: Sessions, saving locale
-
 Migrating Between Session Handlers
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