Fix all wrong instances of "it's".
diff --git a/Doc/Makefile b/Doc/Makefile
index f68e3f2..0d391af 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -87,7 +87,7 @@
 # (e.g. OpenBSD needs package gmake installed; use gmake instead of make)
 PWD=$(shell pwd)
 
-# (The trailing colon in the value is needed; TeX places it's default
+# (The trailing colon in the value is needed; TeX places its default
 # set of paths at the location of the empty string in the path list.)
 TEXINPUTS=$(PWD)/commontex: