📝 Fix RST mdash typos @ 3.1 changelog summary

Missing whitespaces confuse RST parsers and they fail to recognize
that em dashes don't need to remain double hyphens.

Author: webknjaz

doc/whatsnew/3/3.1/index.rst

@@ -12,7 +12,7 @@
 Summary -- Release highlights
 =============================
 
-Two new checks--``use-yield-from``, ``deprecated-attribute``--
+Two new checks -- ``use-yield-from``, ``deprecated-attribute`` --
 and a smattering of bug fixes.
 
 .. towncrier release notes start