Escape line ending slashes in doc.

doc/new_language.rst

@@ -189,7 +189,7 @@ As a local constraint:
 
 .. image:: img/local_constraint.png
 
-.. warning:: Multi-line local constraints need to have each line end with "\\".
+.. warning:: Multi-line local constraints need to have each line end with ``\\``.
 
 One disadvantage of this constraint is that the minimum number of instances (also 1) is not checked. A global constraint solves this: