Update abstractions.md - v minor grammar change

It's => it is, does not make sense

Changed, removed unnecessary word

Author: rpcme

content/docs/manual/abstractions.md

@@ -133,7 +133,7 @@ This timestamp is made of 2 items:
   - The lower 32-bit part is a fraction of second, but with the 4 last bits replaced by a counter.
 
   This time gives a theoretical resolution of (0xF * 10^9 / 2^32) = 3.5 nanoseconds.  
-  It's counter part guarantees that the same time cannot be generated twice and that the _happened-before_ relationship is preserved.
+  The counter guarantees that the same time cannot be generated twice and that the _happened-before_ relationship is preserved.
 
 - The **UUID** of the Zenoh router that generated the time.