Fixed typo

Author: pborreli

CONTRIBUTING.md

@@ -48,7 +48,7 @@ The general rule we follow is "use Visual Studio defaults".
 2. We use four spaces of indentation (no tabs)
 3. We use "_camelCase" private members and use "readonly" where possible
 4. We avoid `this.` unless absolutely necessary
-5. We always specify the visiblity, even if it's the default (i.e.
+5. We always specify the visibility, even if it's the default (i.e.
    `private string _foo` not `string _foo`)
 6. Namespace imports should be specified at the top of the file, *outside* of
    `namespace` declarations and should be sorted alphabetically, with `System.`