Documentation fixes for scio parquet testing harness

[johnwalk-spotify]

in the testing section for scio.io.Parquet There are several points where the examples appear out of date or unclear.

1. predicate filtering: the function withPredicate appears to have been renamed to withFilter

2. the sequencing of predicate vs projection is inverted - in the first example (avro projection) we have

records withProjection projection withPredicate predicate

however this fails to compile due to apparently interpreting withFilter as a scala collection method, leading to a type mismatch expecting a method returning a Boolean rather than a parquet FilterPredicate.

Fiipping the order to

records withFilter predicate withProjection projection

should resolve this (tested with the case-class projection approach, which requires other changes - see below)

3. postfix operators do not work for the case-class projection method - in the second example we have

records withProjection[MyProjection]

but this fails with type application is not allowed for postfix operators. Using a normal method call records.withProjection[MyProjection] resolves this.