parser: fix struct field colon after keyword

[kbkpbot]

Fix issue #25380

[kbkpbot]

according to parser code(struct.v):

			} else if p.tok.kind == .key_module {
				if module_pos != -1 {
					p.error('redefinition of `module` section')
					return ast.StructDecl{}
				}
				p.next()
				p.check(.colon)
				module_pos = ast_fields.len
				is_field_pub = false
				is_field_mut = false
				is_field_global = false
			}

then the following code is invalid, as module is a keyword in struct declare...

struct RootAST {
        module []rune
}

and it must change to something like that :

struct RootAST {
        mymodule []rune
}

I did not see that we can use module or __global as access modifiers for struct field in document.