use const references where appropriate

src/utilstrencodings.cpp

@@ -488,7 +488,7 @@ bool ParseDouble(const std::string& str, double *out)
     return text.eof() && !text.fail();
 }
 
-std::string FormatParagraph(const std::string in, size_t width, size_t indent)
+std::string FormatParagraph(const std::string& in, size_t width, size_t indent)
 {
     std::stringstream out;
     size_t col = 0;