doc/source/index.rst: fix tiny, but impactful typo

Fixes #435
11 jobs for master in 31 minutes and 4 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Prepare
passed #76816046
source_dist

00:00:24

 
  Test
passed #76816051
docs

00:14:57

passed #76816047
tests-debian-8

00:27:42

passed #76816048
tests-debian-9

00:26:43

passed #76816049
tests-fedora-27

00:26:57

passed #76816050
tests-unix

00:13:01

 
  Post
passed #76816053
analysis

00:00:38

passed #76816052
allowed to fail
codequality

00:02:57

passed #76816054
coverage

00:00:20

84.84%
passed #76816055
pages

00:00:26

 
  Deploy
passed pages:deploy

00:00:07