Spelling: It's Versus Its

Corrects incorrect usage of contraction for 'it is', when possessive 'its' was required.

Differential Revision: https://developer.blender.org/D9250

Reviewed by Campbell Barton

source/blender/functions/FN_array_spans.hh

@@ -20,7 +20,7 @@
  * \ingroup fn
  *
  * An ArraySpan is a span where every element contains an array (instead of a single element as is
- * the case in a normal span). It's main use case is to reference many small arrays.
+ * the case in a normal span). Its main use case is to reference many small arrays.
  */
 
 #include "FN_spans.hh"