obstack: fix spelling of similar

Noticed using Lucas De Marchi's codespell tool.
Signed-off-by: NStefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Acked-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/obstack.h

@@ -128,7 +128,7 @@ extern "C" {
 
 #define __BPTR_ALIGN(B, P, A) ((B) + (((P) - (B) + (A)) & ~(A)))
 
-/* Similiar to _BPTR_ALIGN (B, P, A), except optimize the common case
+/* Similar to _BPTR_ALIGN (B, P, A), except optimize the common case
    where pointers can be converted to integers, aligned as integers,
    and converted back again.  If PTR_INT_TYPE is narrower than a
    pointer (e.g., the AS/400), play it safe and compute the alignment