Incorrect definition of expressions in grammar

Hello,

Looking through the grammar spec and expressions definitions i noticed
a naming inconsistency that causes some problems in the grammar.

The problem is around the `expression` for instance:

primary-expression:
    variable-name
    qualified-name
    literal
    const-expression
    intrinsic
    anonymous-function-creation-expression
    (  expression  )
    $this

The definition of the  `expression` production is given later under
"script inclusion operators":

expression:
    yield-expression
    include-expression
    include-once-expression
    require-expression
    require-once-expression

My suggestion is that the `expression` symbol under "script inclusion
operators" be renamed to `script-inclusion-expression` and all
references to `expression` in the context of primary expressions be
renamed to `primary-expression`.

Not sure if i am misinterpreting it there but it seems to make much
more sense to me that way. I wanted to get the lists opinion before
putting it together in a PR.

Thanks!
- Ken