doc: fix an infinitely recursing example

doc: fix an infinitely recursing example

The "single" macro infinitely recurses because its expansion contains
the unquoted text " single-". The "double" macro almost has the same
problem, but it is protected by extra quotes. In any case, the macro
names being repeated in the macro definitions is not necessary.

This commit changes the macro names to "foo" and "bar", which are taken
from a very similar example in the GNU M4 1.4.17 manual. See lines 1971
to 1980 of v1.4.17:doc/m4.texi in the GNU M4 Git repository.

* doc/autoconf.texi (Quoting and Parameters): Fix broken example.