meson: Fix typo.

Fixes: 6b4c7047d571 ("meson: build gallium nine state_tracker")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/meson.build b/meson.build
index 86ae13f..34f29cf 100644
--- a/meson.build
+++ b/meson.build
@@ -613,7 +613,7 @@
     error('The nine state tracker requires gallium softpipe/llvmpipe.')
   elif not (with_gallium_radeonsi or with_gallium_nouveau or with_gallium_r600
             or with_gallium_r300 or with_gallium_svga or with_gallium_i915)
-    error('The nine state tracker requires at least on non-swrast gallium driver.')
+    error('The nine state tracker requires at least one non-swrast gallium driver.')
   endif
   if not with_dri3
     error('Using nine with wine requires dri3')