fix(crash) due to calling isalpha with signed char in get_indentatnion()

src/library/documentation.cpp

@@ -82,7 +82,7 @@ static unsigned get_indentation(std::string const & s) {
     unsigned r  = 0;
     bool searching = true;
     unsigned i = 0;
-    for (auto it = s.begin(); it != s.end(); it++) {
+    for (auto it = (const unsigned char*)s.data(), e = (const unsigned char*)s.data() + s.size(); it != e; ++it) {
         if (*it == '\n') {
             i = 0;
             searching = true;