Remove last vestiges of HybridBitSet.

This is in a test where the arrangement of backticks matters, but the
exact words do not.

Author: nnethercote

tests/rustdoc-ui/unescaped_backticks.rs

@@ -218,7 +218,7 @@ pub mod rustc {
     pub fn with_options() {}
 
     /// Subtracts `set from `row`. `set` can be either `BitSet` or
-    /// `HybridBitSet`. Has no effect if `row` does not exist.
+    /// `ChunkedBitSet`. Has no effect if `row` does not exist.
     //~^ ERROR unescaped backtick
     ///
     /// Returns true if the row was changed.

tests/rustdoc-ui/unescaped_backticks.stderr

@@ -124,19 +124,19 @@ LL |     /// also avoids the need to import `OpenOptions\`.
    |                                                    +
 
 error: unescaped backtick
-  --> $DIR/unescaped_backticks.rs:221:46
+  --> $DIR/unescaped_backticks.rs:221:47
    |
-LL |     /// `HybridBitSet`. Has no effect if `row` does not exist.
-   |                                              ^
+LL |     /// `ChunkedBitSet`. Has no effect if `row` does not exist.
+   |                                               ^
    |
 help: a previous inline code might be longer than expected
    |
 LL |     /// Subtracts `set` from `row`. `set` can be either `BitSet` or
    |                       +
 help: if you meant to use a literal backtick, escape it
    |
-LL |     /// `HybridBitSet`. Has no effect if `row\` does not exist.
-   |                                              +
+LL |     /// `ChunkedBitSet`. Has no effect if `row\` does not exist.
+   |                                               +
 
 error: unescaped backtick
   --> $DIR/unescaped_backticks.rs:247:12