Fix spelling of hexadecimal

[sehe]

The library and documentation misspells it (consistently, which is good :)) as hexidecimal. That makes it hard for (new) users to find out by searching documentation.

The fifteen spots fixed:

example/callback_json.cpp|47 col 15| "four hexidecimal digits";
example/callback_json.cpp|49 col 18| "\\uXXXX hexidecimal escape sequence";
example/callback_json.cpp|51 col 38| escape_double_seq = "\\uXXXX hexidecimal escape sequence";
example/json.cpp|57 col 28| // like "expected four hexidecimal digits here:", instead of "expected
example/json.cpp|65 col 15| "four hexidecimal digits";
example/json.cpp|67 col 18| "\\uXXXX hexidecimal escape sequence";
example/json.cpp|69 col 38| escape_double_seq = "\\uXXXX hexidecimal escape sequence";
example/json.cpp|158 col 18| // declare a hexidecimal parser that matches exactly 4.
doc/tutorial.qbk|981 col 18| [ Matches a hexidecimal unsigned integral value. ]
doc/tutorial.qbk|986 col 28| [ Matches exactly the hexidecimal unsigned integral value `_RES_np_(arg0)`. ]
doc/tutorial.qbk|1118 col 42| So, if you wanted to parse exactly eight hexidecimal digits in a row in order
doc/tutorial.qbk|2729 col 18| "\\uXXXX hexidecimal escape sequence";
doc/tutorial.qbk|2731 col 38| escape_double_seq = "\\uXXXX hexidecimal escape sequence";
doc/tutorial.qbk|2741 col 9| put a hexidecimal escape sequence there.
include/boost/parser/parser.hpp|5622 col 13| /** The hexidecimal unsigned integer parser.  Produces an `unsigned int`

The one spot that was inconsistent (outside the gtest subtree):

include/boost/parser/tuple.hpp|66 col 36| // 0xDEADBEEF (hexadecimal)

[tzlaine]

Thanks!