Add missing release to TOC

Author: andrewconnell

docs/toc.yml

@@ -430,6 +430,8 @@
       href: spfx/release-1.9.1.md
     - name: SPFx v1.8.2 - May 7, 2019
       href: spfx/release-1.8.2.md
+    - name: SPFx v1.8.1 - April 16, 2019
+      href: spfx/release-1.8.1.md
     - name: SPFx v1.8.0 - March 14, 2019
       href: spfx/release-1.8.0.md
     - name: SPFx v1.7.1 - December 18, 2018