README: Fix contraction of "it is"

Change-Id: I1dee127a2bf7b5d50086f0bb14300d01a9fe3509
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/README.md b/README.md
index 4c8df81..d944782 100644
--- a/README.md
+++ b/README.md
@@ -167,7 +167,7 @@
 As mentioned above, all new functions must first be added in the testing
 category (using the `LIBPLDM_ABI_TESTING` annotation).
 
-To move a function from the testing category to the stable category, its
+To move a function from the testing category to the stable category, it's
 required that patches demonstrating use of the function in a dependent
 application or library be linked in the commit message of the stabilisation
 change. We require this to demonstrate that the implementer has considered its