Remove the '0' chrono padding modifier from the docs

[dylanpulver]

doc/syntax.md documents a chrono padding modifier that the parser rejects.

The grammar says padding_modifier ::= "-" | "_" | "0" and the table below it
lists a '0' row, for the 11 presentation types the same section names
(H I M S U V W Y d j m). But parse_chrono_format only has cases for '_'
and '-', so every %0 spelling throws format_error: invalid format.

Measured on main (bc82c40), all 11 documented types, std::tm and
std::chrono::seconds, 44 cells:

modifier	OK	error
(none)	11	0
-	11	0
_	11	0
0	0	11

fmt::format("{:%H}",  tm);  // "03"
fmt::format("{:%-H}", tm);  // "3"
fmt::format("{:%_H}", tm);  // " 3"
fmt::format("{:%0H}", tm);  // throws: invalid format

The docs are the stale side, not the code. ca8eeb09 (#3976, 2024-05-30)
added the '0' grammar and table row; 7bd11b5c (2024-06-08, nine days
later) removed case '0' and its tests under "Remove a redundant extension
to reduce divergence from std::format", touching only chrono.h and
chrono-test.cc. Zero padding is the default, so it was redundant — this
just drops the two stale mentions and notes the default.

Docs-only. The "only supported for" list is accurate: all 11 do honor -
and _, checked with values short enough to discriminate.

Drafted with Claude Code (claude-opus-5); measurements and archaeology run
and reviewed by me.

[vitaut]

Thank you!