Update SA1307.md

Author: marcospgp

documentation/SA1307.md

@@ -21,7 +21,7 @@ The name of a public or internal field in C# does not begin with an upper-case l
 
 ## Rule description
 
-A violation of this rule occurs when the name of a public or internal field begins with a lower-case letter. Public or internal fields should being with an upper-case letter.
+A violation of this rule occurs when the name of a public or internal field begins with a lower-case letter. Public or internal fields should begin with an upper-case letter.
 
 If the field or variable name is intended to match the name of an item associated with Win32 or COM, and thus needs to start with a lower-case letter, place the field or variable within a special *NativeMethods* class. A NativeMethods class is any class which contains a name ending in NativeMethods, and is intended as a placeholder for Win32 or COM wrappers. StyleCop will ignore this violation if the item is placed within a NativeMethods class.