[docs] Fix an RST error introduced in e823068.

This should fix the 'Explicit markup ends without a blank line' error seen on
http://lab.llvm.org:8011/builders/llvm-sphinx-docs.

Thanks to Daniel Sanders for spotting this.

Author: lhames

llvm/docs/HowToSetUpLLVMStyleRTTI.rst

@@ -434,7 +434,7 @@ This open-hierarchy RTTI support should only be used if your use case requries
 it. Otherwise the standard LLVM RTTI system should be preferred.
 
 .. _`Curiously Recurring Template Idiom`:
-https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
+  https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
 
 E.g.