doc: mark URL as such in Markdown (#1046)

Unlike GitHub flavored Markdown, in "plain" Markdown URLs
must be marked as such in order to be turned into links.

Author: fingolfin

docs/src/index.md

@@ -58,7 +58,7 @@ Pkg.add("PackageCompiler")
 
 !!! note
     It is strongly recommended to use the official binaries that are downloaded from 
-    https://julialang.org/downloads/. Distribution-provided Julia installations are
+    <https://julialang.org/downloads/>. Distribution-provided Julia installations are
     unlikely to work properly with this package.
 
 To use PackageCompiler a C-compiler needs to be available: