Haiku Book: fix some typos.

The possessive form of "it" is "its", not "it's" ("it's" is just
shorthand for "it is").

Closes #11712.

docs/user/interface/Font.dox

@@ -32,7 +32,7 @@
 /*!
 	\var B_STRING_SPACING
 
-	Optimizes the position of each character within it's space. Collisions
+	Optimizes the position of each character within its space. Collisions
 	are unlikely but characters may touch each other. Best mode to use when
 	the screen needs to match what appears on the printed page.
 

docs/user/interface/View.dox

@@ -3712,7 +3712,7 @@ SetViewColor(Parent()->ViewColor());
 	\brief Adds \a child to the view hierarchy immediately before \a before.
 
 	A view may only have one parent at a time so \a child must not have already
-	been added to the view hierarchy. if \a before \c NULL then \a child is added
+	been added to the view hierarchy. If \a before is \c NULL then \a child is added
 	to the end of the tree. If the view is attached to a window \a child and all
 	of its descendent views also become attached to the window invoking an
 	AttachedToWindow() method on each view.