Fix misleading summary in docs for FcStrStrIgnoreCase

Reported by Jonathan Kew

Fixes fontconfig/fontconfig#146
2 jobs for master in 16 minutes and 10 seconds (queued for 87 minutes and 58 seconds)
Status Job ID Name Coverage
  Build
passed #174607
shared-build

00:07:44

passed #174608
static-build

00:08:25