sdsrange() does not need to return a value.

Actaully the string is modified in-place and a reallocation is never
needed, so there is no need to return the new sds string pointer as
return value of the function, that is now just "void".

src/sds.h

@@ -77,7 +77,7 @@ sds sdscatprintf(sds s, const char *fmt, ...);
 #endif
 
 sds sdstrim(sds s, const char *cset);
-sds sdsrange(sds s, int start, int end);
+void sdsrange(sds s, int start, int end);
 void sdsupdatelen(sds s);
 void sdsclear(sds s);
 int sdscmp(const sds s1, const sds s2);