Make it clear this is boost at index time (elastic#51390)

The way it was originally written, it sounds like 
we are boosting at query time.
 Of course, the effect is at query time, 
but the point here is that boosting is done at index time

docs/reference/mapping/params/boost.asciidoc

@@ -1,8 +1,7 @@
 [[mapping-boost]]
 === `boost`
 
-Individual fields can be _boosted_ automatically -- count more towards the relevance score
--- at query time, with the `boost` parameter as follows:
+Individual fields can be _boosted_ automatically at index time (to count more towards the relevance score), with the `boost` parameter as follows:
 
 [source,console]
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