Fix the bug of memory release in documents.

README.md

@@ -274,5 +274,6 @@ braudrate:9600
     print(t.read(nil,t.C))
     -- Don't forget to release it after use
     t = nil
+	ds18b20 = nil
     package.loaded["ds18b20"]=nil   
 ```

lua_modules/ds18b20/ds18b20.CN.md

@@ -109,6 +109,7 @@ print(t.readNumber(nil,t.F))
 t1, t2 = t.readNumber()
 -- Don't forget to release it after use
 t = nil
+ds18b20 = nil
 package.loaded["ds18b20"]=nil
 
 ```
@@ -151,6 +152,7 @@ print(t.read())
 print(t.read(nil,t.C))
 -- Don't forget to release it after use
 t = nil
+ds18b20 = nil
 package.loaded["ds18b20"]=nil
 ```
 ####参见

lua_modules/ds18b20/ds18b20.EN.md

@@ -107,6 +107,7 @@ print(t.readNumber(nil,t.F))
 t1, t2 = t.readNumber()
 -- Don't forget to release it after use
 t = nil
+ds18b20 = nil
 package.loaded["ds18b20"]=nil
 ```
 ####See also
@@ -148,6 +149,7 @@ print(t.read())
 print(t.read(nil,t.C))
 -- Don't forget to release it after use
 t = nil
+ds18b20 = nil
 package.loaded["ds18b20"]=nil
 ```
 ####See also