Fix typo.

Reported-by: Antonin Houska
Author: Antonin Houska
Backpatch-through: 11, where it was introduced
Discussion: https://postgr.es/m/2246.1578900133@antos

src/include/access/session.h

@@ -19,7 +19,7 @@ struct SharedRecordTypmodRegistry;
 
 /*
  * A struct encapsulating some elements of a user's session.  For now this
- * manages state that applies to parallel query, but it principle it could
+ * manages state that applies to parallel query, but in principle it could
  * include other things that are currently global variables.
  */
 typedef struct Session