correct prefix in tests

Author: MichaelChirico

inst/tests/tests.Rraw

@@ -21161,4 +21161,4 @@ test(2317.9, DT1[DF2, on='a', .(e = x.a + i.e)]$e, 6)
 # Test for incompatible factor joins rephrased by merge.data.table(#7048)
 DT1=data.table(a=factor('a'))
 DT2=data.table(a=1L)
-test(2318, merge(DT1, DT2, by = "a"), error = "Incompatible join types: x.a (factor) and i.a (integer). Factor columns must join to factor or character columns.")
+test(2318, merge(DT1, DT2, by = "a"), error = "Incompatible join types: x.a (factor) and y.a (integer). Factor columns must join to factor or character columns.")