Fixed spelling of logical AND operator, no functional changes.

Found by PVS-Studio.
diff --git a/src/core/ngx_output_chain.c b/src/core/ngx_output_chain.c
index f784578..7f5dc78 100644
--- a/src/core/ngx_output_chain.c
+++ b/src/core/ngx_output_chain.c
@@ -512,7 +512,7 @@
     size = ngx_buf_size(src);
     size = ngx_min(size, dst->end - dst->pos);
 
-    sendfile = ctx->sendfile & !ctx->directio;
+    sendfile = ctx->sendfile && !ctx->directio;
 
 #if (NGX_SENDFILE_LIMIT)