Fixed issue with string concatenation

C++17 support for [[nodiscard]] raised a case of usage of string
concatenation (+ operator) discarding the result, causing an error.
This commit changes the + operator with a +=.

Author: Enrico Steffinlongo

src/goto-instrument/document_properties.cpp

@@ -238,7 +238,7 @@ document_propertiest::get_code(const source_locationt &source_location)
       while(line_no.size()<4)
         line_no=" "+line_no;
 
-      line_no+"  ";
+      line_no+="  ";
 
       tmp+=escape_latex(it->text, true);
 
@@ -251,7 +251,7 @@ document_propertiest::get_code(const source_locationt &source_location)
       while(line_no.size()<4)
         line_no="&nbsp;"+line_no;
 
-      line_no+"&nbsp;&nbsp;";
+      line_no+="&nbsp;&nbsp;";
 
       tmp+=escape_html(it->text);