Tests: fixed spelling errors in README.

README

diff --git a/README b/README
index b9794fc..e5fe008 100644
--- a/README
+++ b/README
@@ -25,12 +25,12 @@
 
 TEST_NGINX_LEAVE
 
-    If set temporary directory with configs and logs won't be deleted on test
-    completion.  Usefull for debugging.
+    If set, temporary directory with configs and logs won't be deleted on test
+    completion.  Useful for debugging.
 
 TEST_NGINX_CATLOG
 
-    Cat error log to stdout after test completion.  Usefull for debugging.
+    Cat error log to stdout after test completion.  Useful for debugging.
 
 TEST_NGINX_UNSAFE