Replace eplxicit max aggregate size of 50 with reference to

BOOST_PARSER_MAX_AGGREGATE_SIZE.

Partially addresses #51.

doc/tutorial.qbk

@@ -581,7 +581,7 @@ attribute type is a tuple, you can also pass a tuple as an out-param to
 [important This automatic use of `struct`s as if they were tuples depends on a
 bit of metaprogramming.  Due to compiler limits, the metaprogram that detects
 the number of data members of a `struct` is limited to a maximum number of
-members.  Fortunately, that limit is pretty high _emdash_ 50 members.]
+members.  Fortunately, that limit is configurable; see _AGGR_SIZE_.]
 
 [endsect]