Update variables.c

Aligned patch 5 (this) with patch 4 to ease readability. In this case, "if (isalpha(c) && !isupper(c))" has been simplified to "if (islower(c))"

Author: sadeem-albir

chapter_4/exercise_4_06/variables.c

@@ -251,7 +251,7 @@ int getop(char s[])
 
   s[1] = '\0';
 
-  if (isalpha(c) && !isupper(c))
+  if (islower(c))
   {
     var = c;
     return VARGET;