README: Made line break between quotation and Bjarne's name actually rendered (isocpp#2318)

The line break existed in source code and obviously meant to be there, just wasn't rendered because Markdown, like HTML, ignores whitespace (including line breaks) by default. Two spaces at the end of the pre-line-break line allow to indicate an actual line break visible in the rendered Markdown document.

Author: Marat-Tanalin

README.md

@@ -1,6 +1,6 @@
 [![C++ Core Guidelines](cpp_core_guidelines_logo_text.png)](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines)
 
->"Within C++ is a smaller, simpler, safer language struggling to get out."
+>"Within C++ is a smaller, simpler, safer language struggling to get out."  
 >-- <cite>Bjarne Stroustrup</cite>
 
 The [C++ Core Guidelines](CppCoreGuidelines.md) are a collaborative effort led by Bjarne Stroustrup, much like the C++ language itself. They are the result of many