don't capitalize Undefined Behavior

src/behavior-considered-undefined.md

@@ -101,7 +101,7 @@ alignment 1). In other words, the alignment requirement derives from the type of
 the pointer that was dereferenced, *not* the type of the field that is being
 accessed.
 
-Note that a place based on a misaligned pointer only leads to Undefined Behavior
+Note that a place based on a misaligned pointer only leads to undefined behavior
 when it is loaded from or stored to. `addr_of!`/`addr_of_mut!` on such a place
 is allowed. `&`/`&mut` on a place requires the alignment of the field type (or
 else the program would be "producing an invalid value"), which generally is a

src/expressions/operator-expr.md

@@ -100,7 +100,7 @@ struct Packed {
 }
 
 let packed = Packed { f1: 1, f2: 2 };
-// `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
+// `&packed.f2` would create an unaligned reference, and thus be undefined behavior!
 let raw_f2 = ptr::addr_of!(packed.f2);
 assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
 ```
@@ -116,7 +116,7 @@ struct Demo {
 
 let mut uninit = MaybeUninit::<Demo>::uninit();
 // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
-// and thus be Undefined Behavior!
+// and thus be undefined behavior!
 let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
 unsafe { f1_ptr.write(true); }
 let init = unsafe { uninit.assume_init() };

src/types/textual.md

@@ -4,15 +4,15 @@ The types `char` and `str` hold textual data.
 
 A value of type `char` is a [Unicode scalar value] (i.e. a code point that is
 not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF
-or 0xE000 to 0x10FFFF range. It is immediate [Undefined Behavior] to create a
+or 0xE000 to 0x10FFFF range. It is immediate [undefined behavior] to create a
 `char` that falls outside this range. A `[char]` is effectively a UCS-4 / UTF-32
 string of length 1.
 
 A value of type `str` is represented the same way as `[u8]`, a slice of
 8-bit unsigned bytes. However, the Rust standard library makes extra assumptions
 about `str`: methods working on `str` assume and ensure that the data in there
 is valid UTF-8. Calling a `str` method with a non-UTF-8 buffer can cause
-[Undefined Behavior] now or in the future.
+[undefined behavior] now or in the future.
 
 Since `str` is a [dynamically sized type], it can only be instantiated through a
 pointer type, such as `&str`.
@@ -26,5 +26,5 @@ Every byte of a `char` is guaranteed to be initialized (in other words,
 some bit patterns are invalid `char`s, the inverse is not always sound).
 
 [Unicode scalar value]: http://www.unicode.org/glossary/#unicode_scalar_value
-[Undefined Behavior]: ../behavior-considered-undefined.md
+[undefined behavior]: ../behavior-considered-undefined.md
 [dynamically sized type]: ../dynamically-sized-types.md