fix: compiler warning

Author: ohkimur

chapter_1/exercise_1_13/histogram.c

@@ -23,7 +23,7 @@ int main(void)
   // specific index
   char c;
   int word_count_index = 0;
-  while (c = getchar())
+  while ((c = getchar()))
   {
     if (c == ' ' || c == '\t' || c == '\n' || c == EOF)
     {
@@ -43,10 +43,10 @@ int main(void)
 
         word_count_index = 0;
       }
-      if (c == EOF) 
+      if (c == EOF)
       {
         break;
-      }  
+      }
     }
     else
     {