[doc] Fix a typo in the graphlib docs (#22030)

diff --git a/Doc/library/graphlib.rst b/Doc/library/graphlib.rst
index 820615e..0faca21 100644
--- a/Doc/library/graphlib.rst
+++ b/Doc/library/graphlib.rst
@@ -121,7 +121,7 @@
           if ts.is_active():
               ...
 
-      if possible to simply do::
+      it is possible to simply do::
 
           if ts:
               ...