webpmux.1: correct grammar
webpmux.1: correct grammar

argument -> arguments

Change-Id: I57132c942150f18dccc5ef84c4c09a6dcc054ffa

webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp