patch 8.1.0528: various typos in comments

Problem:    Various typos in comments.
Solution:   Fix the typos.
diff --git a/src/if_tcl.c b/src/if_tcl.c
index 2289365..7fb9174 100644
--- a/src/if_tcl.c
+++ b/src/if_tcl.c
@@ -113,7 +113,7 @@
 
 /*
  *  List of Tcl interpreters who reference a vim window or buffer.
- *  Each buffer and window has it's own list in the w_tcl_ref or b_tcl_ref
+ *  Each buffer and window has its own list in the w_tcl_ref or b_tcl_ref
  *  struct member.  We need this because Tcl can create sub-interpreters with
  *  the "interp" command, and each interpreter can reference all windows and
  *  buffers.