Compose.man: fix escaped hexadecimal char description

The man page says:
    Strings may be direct text encoded in the locale for which the
    compose file is to be used, or an escaped octal or hexadecimal
    character code.   Octal codes are specified as "\123" and
    hexadecimal codes as "\0x123a".

But the grammar in the parser and the implementation say:
    ESCAPED_CHAR  ::= ('\\' | '\"' | OCTAL | HEX )
    HEX           ::= '\' (x|X) HEX_CHAR [HEX_CHAR]]
    HEX_CHAR      ::= (0|1|2|3|4|5|6|7|8|9|A|B|C|D|E|F|a|b|c|d|e|f)

So "\0x123a" -> "\x3a".

Signed-off-by: Ran Benita <ran234@gmail.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>