configure.ac: Fix help string for --disable-pwr8 configure option

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
diff --git a/configure.ac b/configure.ac
index df3eb6b..5c460f4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -455,7 +455,7 @@
 CFLAGS=$save_CFLAGS
 
 AC_ARG_ENABLE(pwr8,
-   [AS_HELP_STRING([--disable-pwr8-inst],
+   [AS_HELP_STRING([--disable-pwr8],
                    [disable POWER8-specific instructions])],
    [enable_pwr8=$enableval], [enable_pwr8=auto])