[docs] Use = over ::= for EBNF

[jaki]

According to Wikipedia, = is used for definitions over ::=. Our .ebnf files have a majority of the rules using ::= and a few rules using =. They both work, and the generator converts them to ::=. Therefore, there is currently no difference to the end user. But to those looking at the source EBNF file, using = could make syntax highlighting work properly.

[bllewell]

Fixed a long time ago. Forgot to close this until now.