man/img2webp.1: update date
man/img2webp.1: update date

after:
a80954a1 Reword img2webp synopsis command line

Change-Id: I0a5c18f7947d15fcd66dc32fab9b525186f7fcc5

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