Remove incorrect link to abstract syntax

When referring to the `rec` keyword, do not link to the abstract syntax that
happens to have the same name. Fix suggested by @bvisness and split out from
#413.

Do not link to the abstract syntax

document/core/text/types.rst

@@ -263,7 +263,7 @@ Recursive Types
 Abbreviations
 .............
 
-Singular recursive types can omit the |Trec| keyword:
+Singular recursive types can omit the :math:`\text{rec}` keyword:
 
 .. math::
  \begin{array}{llclll}