_s: Remove leading white space for theme name.

At some point in the past it was believed that a leading whitespace was
necessary for a proper replacement.

Let’s see if the existing replace handler can indeed handle it without
it.

See
Automattic/underscores.me@7671cad5685ae8df50a0
2f871a1ae28119b8dbf1

style.css

@@ -13,7 +13,7 @@ Tags:
 This theme, like WordPress, is licensed under the GPL.
 Use it to make something cool, have fun, and share what you've learned with others.
 
- _s is based on Underscores http://underscores.me/, (C) 2012-2014 Automattic, Inc.
+_s is based on Underscores http://underscores.me/, (C) 2012-2014 Automattic, Inc.
 
 Resetting and rebuilding styles have been helped along thanks to the fine work of
 Eric Meyer http://meyerweb.com/eric/tools/css/reset/index.html