docs: fix reference to #noibss in v2018.2.1 release notes

Uncovered by sphinx-build:
docs/releases/v2018.2.1.rst:19: WARNING: Too many autonumbered footnote references: only 0 corresponding footnotes available.
docs/releases/v2018.2.1.rst:19: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:26: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:30: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:34: WARNING: Unknown target name: "noibss".
docs/releases/v2018.2.1.rst:35: WARNING: Unknown target name: "noibss".