RISC-V: Use "@minus{}2 GB" instead of "-2 GB" in invoke.texi

RISC-V: Use "@minus{}2 GB" instead of "-2 GB" in invoke.texi

gcc/ChangeLog

2017-11-01  Palmer Dabbelt  <palmer@dabbelt.com>

	* doc/invoke.texi (RISC-V Options): Use "@minus{}2 GB", not "-2 GB".

From-SVN: r254331