Fix a couple of index entries in the manual.

[Ralf Wildenhues]

Pushed as obvious.

Cheers,
Ralf

    Fix a couple of index entries in the manual.
    
    * doc/autoconf.texi (Polymorphic Variables): Fix index entries
    for AS_VAR_APPEND, AS_VAR_ARITH.

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 4685972..db8c88e 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -13161,7 +13161,7 @@ Polymorphic Variables
 @end defmac
 
 @defmac AS_VAR_APPEND (@var{var}, @var{text})
address@hidden
address@hidden
 Emit shell code to append the shell expansion of @var{text} to the end
 of the current contents of the polymorphic shell variable @var{var},
 taking advantage of shells that provide the @samp{+=} extension for more
@@ -13186,7 +13186,7 @@ Polymorphic Variables
 @end defmac
 
 @defmac AS_VAR_ARITH (@var{var}, @var{expression})
address@hidden
address@hidden
 Emit shell code to compute the arithmetic expansion of @var{expression},
 assigning the result as the contents of the polymorphic shell variable
 @var{var}.  The code takes advantage of shells that provide @samp{$(())}