[Doc] embind.rst - remove "Optional" arg from register_optional

[CharlieFRuan]

Calling register_optional<std::string>("Optional"); results in:

error: no matching function for call to 'register_optional'
    8 |   emscripten::register_optional<std::string>("Optional");
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/path/to/emsdk/upstream/emscripten/cache/sysroot/include/emscripten/bind.h:1880:6: note: candidate function template not viable: requires 0 arguments, but 1 was provided
 1880 | void register_optional() {
      |      ^
1 error generated

As shown in later in the doc and in usage in #21076, register_optional should not take in an argument. Also see the source code:

emscripten/system/include/emscripten/bind.h

Line 2006 in df22009

void register_optional() {

[CharlieFRuan]

Hmm not sure how to rerun the test -- I think the test failing shouldn't relate to the change in this PR which is just a one-line change of the doc

[brendandahl]

Those are unrelated failures due to some output size tests changing.