Fix: add namespace core at file scope in core_snippets.cpp

write_string() uses core::string_view at doc_core_snippets namespace
scope, but namespace core was only declared inside fxx(). namespace net
works globally because config.hpp defines it at global scope; core
needs an equivalent declaration. Add it at file scope alongside the
existing using namespace boost::beast.

Author: ssam18

test/doc/core_snippets.cpp

@@ -19,6 +19,7 @@
 //]
 
 using namespace boost::beast;
+namespace core = boost::core;   // from <boost/beast.hpp>
 
 namespace doc_core_snippets {