FYI: doc grammar nits

[Jim Meyering]

I've just fixed these grammar nits:

        * doc/autoconf.texi: Adjust grammar around use of "heuristics".

Index: doc/autoconf.texi
===================================================================
RCS file: /sources/autoconf/autoconf/doc/autoconf.texi,v
retrieving revision 1.1135
diff -u -p -r1.1135 autoconf.texi
--- doc/autoconf.texi   9 Mar 2007 19:51:24 -0000       1.1135
+++ doc/autoconf.texi   17 Mar 2007 09:00:49 -0000
@@ -6137,7 +6137,7 @@ AC_CHECK_TYPES([unsigned long long int,

 Autoconf, up to 2.13, used to provide to another version of
 @code{AC_CHECK_TYPE}, broken by design.  In order to keep backward
-compatibility, a simple heuristics, quite safe but not totally, is
+compatibility, a simple heuristic, quite safe but not totally, is
 implemented.  In case of doubt, read the documentation of the former
 @code{AC_CHECK_TYPE}, see @ref{Obsolete Macros}.

@@ -16829,7 +16829,7 @@ AC_CHECK_TYPE(address@hidden, [],
 @end example

 In order to keep backward compatibility, the two versions of
address@hidden are implemented, selected by a simple heuristics:
address@hidden are implemented, selected using these heuristics:

 @enumerate
 @item