Make sure Sockets page has h1 header (#26315)

mortenpi · fredrikekre · commit 1a870b466d83 · 2018-03-04T12:08:58.000+01:00
Currently the "Sockets" page shows up as a dash in the navbar of the manual since Documenter is unable to figure out the page title.
diff --git a/stdlib/Sockets/docs/src/index.md b/stdlib/Sockets/docs/src/index.md
@@ -1,4 +1,4 @@
-## Sockets
+# Sockets
 
 ```@meta
 DocTestSetup = :(using Sockets)
