Add a missing apostrophe (let us ~> let's) (#23905)

docs/reference/docs/update.asciidoc

@@ -13,7 +13,7 @@ removes some network roundtrips and reduces chances of version conflicts
 between the get and the index. The `_source` field needs to be enabled
 for this feature to work.
 
-For example, lets index a simple doc:
+For example, let's index a simple doc:
 
 [source,js]
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