Get rid of some remaining references to rlp.PrematureFinalizationError

doc/rlp.md

@@ -88,10 +88,9 @@ with the `initRlpWriter` proc. This should be followed by one or more calls
 to `append` which is overloaded to accept arbitrary values. Finally, you can
 call `finish` to obtain the final `BytesRange`.
 
-If the end result should by a RLP list of particular length, you can replace
+If the end result should be a RLP list of particular length, you can replace
 the initial call to `initRlpWriter` with `initRlpList(n)`. Calling `finish`
-before writing a sufficient number of elements will then result in a
-`PrematureFinalizationError`.
+before writing the sufficient number of elements will then result in an assertion failure.
 
 As an alternative short-cut, you can also call `encode` on an arbitrary value
 (including sequences and user-defined types) to execute all of the steps at

tests/rlp/test_api_usage.nim

@@ -30,7 +30,7 @@ test "you cannot finish a list without appending enough elements":
   writer.append "foo"
   writer.append "bar"
 
-  expect PrematureFinalizationError:
+  expect Defect:
     let result = writer.finish
 
 proc withNewLines(x: string): string = x & "\n"