manual: remove mention of >>>.

doc/rust.md

@@ -1861,7 +1861,7 @@ The former is just more terse and familiar.
 
 #### Compound assignment expressions
 
-The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, `>>`, and `>>>`
+The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>`
 operators may be composed with the `=` operator. The expression `lval
 OP= val` is equivalent to `lval = lval OP val`. For example, `x = x +
 1` may be written as `x += 1`.
@@ -1877,7 +1877,7 @@ from strong to weak:
 * / %
 as
 + -
-<< >> >>>
+<< >>
 &
 ^
 |