Fix incorrect use of compile_fail

`compile_fail` should only be used when the code is meant to show
what *not* to do. In other words, there should be a fundamental flaw
in the code. However, in this case, the example is just incomplete,
so we should use `ignore` to avoid confusing readers.

library/std/src/process.rs

@@ -171,7 +171,7 @@ pub struct Child {
     /// The handle for writing to the child's standard input (stdin), if it
     /// has been captured. You might find it helpful to do
     ///
-    /// ```compile_fail,E0425
+    /// ```ignore (incomplete)
     /// let stdin = child.stdin.take().unwrap();
     /// ```
     ///
@@ -183,7 +183,7 @@ pub struct Child {
     /// The handle for reading from the child's standard output (stdout), if it
     /// has been captured. You might find it helpful to do
     ///
-    /// ```compile_fail,E0425
+    /// ```ignore (incomplete)
     /// let stdout = child.stdout.take().unwrap();
     /// ```
     ///
@@ -195,7 +195,7 @@ pub struct Child {
     /// The handle for reading from the child's standard error (stderr), if it
     /// has been captured. You might find it helpful to do
     ///
-    /// ```compile_fail,E0425
+    /// ```ignore (incomplete)
     /// let stderr = child.stderr.take().unwrap();
     /// ```
     ///