compat/regex: fix spelling and grammar in comments

Some of these were found using Lucas De Marchi's codespell tool.
Others noticed by Eric Sunshine.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/compat/regex/regex_internal.c b/compat/regex/regex_internal.c
index 193854c..d4121f2 100644
--- a/compat/regex/regex_internal.c
+++ b/compat/regex/regex_internal.c
@@ -1284,7 +1284,7 @@
 
 /* Insert the new element ELEM to the re_node_set* SET.
    SET should not already have ELEM.
-   return -1 if an error is occured, return 1 otherwise.  */
+   return -1 if an error has occurred, return 1 otherwise.  */
 
 static int
 internal_function
@@ -1341,7 +1341,7 @@
 
 /* Insert the new element ELEM to the re_node_set* SET.
    SET should not already have any element greater than or equal to ELEM.
-   Return -1 if an error is occured, return 1 otherwise.  */
+   Return -1 if an error has occurred, return 1 otherwise.  */
 
 static int
 internal_function
@@ -1416,7 +1416,7 @@
 
 
 /* Add the token TOKEN to dfa->nodes, and return the index of the token.
-   Or return -1, if an error will be occured.  */
+   Or return -1, if an error has occurred.  */
 
 static int
 internal_function