DOC: lua: fix Sphinx warning from core.get_var()

Since f034139bc0 ("MINOR: lua: Allow reading "proc." scoped vars from
LUA core."), a new Sphinx warning is emitted when generating the lua doc:

  "WARNING: Field list ends without a blank line; unexpected unindent."

This is due to a missing space after the line break continuation, sphinx
parser is very restrictive unfortunately!

Suppressing the warning and fixing the html output at the same time by
adding the missing space.

doc/lua-api/index.rst

@@ -389,7 +389,7 @@ Core class
   This is limited to "proc." scoped variables.
 
   :param string var: The variable name in "proc." scope according with the
-  HAProxy variable syntax.
+   HAProxy variable syntax.
 
 .. js:function:: core.now()