Fix two typos in coding style

site/dev/contrib/style.md

 Coding Style Guidelines
 =======================
 
 These conventions have evolved over time. Some of the earlier code in both
 projects doesn’t strictly adhere to the guidelines. However, as the code evolves
 we hope to make the existing code conform to the guildelines.
 
 Files
 -----
 
@@ skipping 358 matching lines @@
     ...
 };
 ~~~~
 
 Method calls within method calls should be prefixed with dereference of the 
 'this' pointer. For example:
 
 <!--?prettify?-->
 ~~~~
 this->method();
-Memory Managemt
+Memory Management
 ~~~~
 
 All memory allocation should be routed through SkNEW and its variants. These are
 #defined in SkPostConfig.h, but the correct way to get access to the config
 system is to #include SkTypes.h, which will allow external users of the library
 to provide a custom memory manager or other adaptations.
 
 <!--?prettify?-->
 ~~~~
 SkNEW(type_name)
@@ skipping 59 matching lines @@
 for packing or performance reasons.
 
 NULL, 0
 -------
 
 Use NULL for pointers, 0 for ints. We prefer explicit NULL comparisons when
 checking for NULL pointers (as documentation):
 
 <!--?prettify?-->
 ~~~~
-if (NULL == x) {  // slightly preferred over if (x)
+if (NULL == x) {  // slightly preferred over if (!x)
    ...
 }
 ~~~~
 
 When checking non-NULL pointers explicit comparisons are not required because it
 reads like a double negative:
 
 <!--?prettify?-->
 ~~~~
 if (x) {  // slightly preferred over if (NULL != x)
@@ skipping 81 matching lines @@
     this->drawBitmapRectToRect(
             bitmap, NULL, dst, paint, kNone_DrawBitmapRectFlag);
 }
 ~~~~
 
 Python
 ------
 
 Python code follows the [Google Python Style Guide](http://google-styleguide.googlecode.com/svn/trunk/pyguide.html).