Fix grammar

vignettes/perform_mr.Rmd

@@ -613,7 +613,7 @@ The current plots being generated are not necessarily adequate because while the
 
 ### Using your own summary data
 
-If you want to perform analysis with your local summary data (i.e. not in the OpenGWAS database) then use then look up the `mv_extract_exposures_local()` function in replace of the `mv_extract_exposures()` function.
+If you want to perform analysis with your local summary data (i.e. not in the OpenGWAS database) then use then look up the `mv_extract_exposures_local()` function instead of the `mv_extract_exposures()` function.
 
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