BUILDING.md: Fix "... OR ..." indentation again

<sigh> GitHub doesn't render indented text the same as my local MarkDown
viewer (MacDown), so it's necessary to indent "... OR ..." by 3 spaces
so both will display it on the same indentation level as "Visual C++
2005 or later" and "MinGW".

Author: dcommander

BUILDING.md

@@ -484,7 +484,7 @@ Build Requirements
     SDK.  You can pass optional arguments to `SetEnv.cmd` to specify a 32-bit
     or 64-bit build environment.
 
-    ... OR ...
+   ... OR ...
 
 - MinGW