Update 04-g-physical-modelling.md

[zeffii]

example_04G11_barmodel.csd

This is another conversion error i think " leftovers from latex special-char escaping " ( https://github.com/csound-flossmanual/csound-floss/issues/49#issuecomment-701582588 )

Here is what it looks like in the pdf, (just the area of the syntax errors) .

but it should look like (without slashes), from csoundQT

confirmed, without the slashes it works.

[zeffii]

there are also various typos i'd like to fix (in the totality of the document), but i'll save that for a larger PR later. unless there is no desire for those things.

[joachimheintz]

sure typo corrections are very much welcome (or even style corrections if you like). thanks!

[zeffii]

thank you @joachimheintz for everything you've done for this manual / and examples . top stuff.