update minimal documenter version

Jutho · Jutho · commit 995b11df123a · 2019-03-06T12:38:54.000+01:00
diff --git a/docs/Project.toml b/docs/Project.toml
@@ -2,4 +2,4 @@
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 
 [compat]
-Documenter = "~0.20"
+Documenter = "~0.21"
