[gram.dcl] assignment-expression -> alignment-expression

[sdutoit]

From an email to cxxeditor:

In N3485 [gram.dcl] the following typo is still present:

- alignas ( alignment-expression ...opt )
+ alignas ( assignment-expression ...opt )

This was fixed in [dcl.attr.grammar], but not reflected in [gram.dcl]

Related: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#1323

Related: https://groups.google.com/a/isocpp.org/forum/?fromgroups=#!topic/std-discussion/K5t9ac8yZSU

[sdutoit]

Fixed in 835cb50