read-cache.c: typofix in comment

Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1527,7 +1527,7 @@ int write_index(const struct index_state *istate, int newfd)
 
 /*
  * Read the index file that is potentially unmerged into given
- * index_state, dropping any unmerged entries.  Returns true is
+ * index_state, dropping any unmerged entries.  Returns true if
  * the index is unmerged.  Callers who want to refuse to work
  * from an unmerged state can call this and check its return value,
  * instead of calling read_cache().