Fix destroy spelling (#63)

diff --git a/lib/skywalking/client.lua b/lib/skywalking/client.lua
index 9efaa43..783cf64 100644
--- a/lib/skywalking/client.lua
+++ b/lib/skywalking/client.lua
@@ -67,7 +67,7 @@
 end
 
 -- Stop the tracing report timer and clean unreported data
-function Client:destoryBackendTimer()
+function Client:destroyBackendTimer()
     self.stopped = true
 
     local metadata_buffer = ngx.shared.tracing_buffer
diff --git a/t/client.t b/t/client.t
index 1321f46..172d16d 100644
--- a/t/client.t
+++ b/t/client.t
@@ -77,14 +77,14 @@
 
 
 
-=== TEST 2: destory backend timer
+=== TEST 2: destroy backend timer
 --- config
     location /t {
         content_by_lua_block {
             local client = require("skywalking.client")
             client.backendTimerDelay = 0.01
             client:startBackendTimer("http://127.0.0.1:" .. ngx.var.server_port)
-            local ok, err = client:destoryBackendTimer()
+            local ok, err = client:destroyBackendTimer()
             ngx.sleep(0.02)
             if not err then
                 ngx.say(ok)