configure.ac: Fix typo in help string.

Using '--without-webrtc-ap' would still attempt to build the
webrtc-based echo canceler.  Adjust the help string of the option to
match the actual option name.

* configure.ac (webrtcap): Help text changed from --without-webrtc-ap
to --without-webrtcap.

Change-Id: I46a30dc87963ca67b0b135fed08523f16834f715

configure.ac

@@ -548,7 +548,7 @@ if test "${SYS}" = "linux"; then
   dnl Check for webrtc-audio-processing development package
   dnl - name: libwebrtc-audio-processing-dev
   AC_ARG_WITH([webrtcap],
-    [AS_HELP_STRING([--without-webrtc-ap],
+    [AS_HELP_STRING([--without-webrtcap],
        [disable support for webrtc-audio-processing])],
     [], [with_webrtc_ap=yes])