Fix doc issues in deprecation change (#9336)

Follow-up to #9309.

The issue in the changelog broke the docs build with:

doc/en/_changelog_towncrier_draft.rst:47: WARNING: Inline literal start-string
without end-string.

The second change isn't as critical, but caused the text to be rendered as
monospace including the tilde (i.e. `~pytest.PytestDeprecationwarning`).

Author: The-Compiler

changelog/9308.breaking.rst

@@ -1,7 +1,7 @@
 **PytestRemovedIn7Warning deprecation warnings are now errors by default.**
 
 Following our plan to remove deprecated features with as little disruption as
-possible, all warnings of type ``PytestRemovedIn7Warning `` now generate errors
+possible, all warnings of type ``PytestRemovedIn7Warning`` now generate errors
 instead of warning messages by default.
 
 **The affected features will be effectively removed in pytest 7.1**, so please consult the

doc/en/backwards-compatibility.rst

@@ -22,7 +22,7 @@ b) transitional: the old and new API don't conflict
 
    We will only start the removal of deprecated functionality in major releases (e.g. if we deprecate something in 3.0 we will start to remove it in 4.0), and keep it around for at least two minor releases (e.g. if we deprecate something in 3.9 and 4.0 is the next release, we start to remove it in 5.0, not in 4.0).
 
-   A deprecated feature scheduled to be removed in major version X will use the warning class `PytestRemovedInXWarning` (a subclass of `~pytest.PytestDeprecationwarning`).
+   A deprecated feature scheduled to be removed in major version X will use the warning class `PytestRemovedInXWarning` (a subclass of :class:`~pytest.PytestDeprecationwarning`).
 
    When the deprecation expires (e.g. 4.0 is released), we won't remove the deprecated functionality immediately, but will use the standard warning filters to turn `PytestRemovedInXWarning` (e.g. `PytestRemovedIn4Warning`) into **errors** by default. This approach makes it explicit that removal is imminent, and still gives you time to turn the deprecated feature into a warning instead of an error so it can be dealt with in your own time. In the next minor release (e.g. 4.1), the feature will be effectively removed.