Replace the trailing markdown spaces by a backslash

They are equivalent according to
<https://spec.commonmark.org/0.29/#hard-line-breaks>,
and the trailing spaces tend to be a pain (because the make git
complain, editors tend to want to remove them − the `.editorconfig`
actually specifies that − etc..).

src/libstore/globals.hh

@@ -701,7 +701,7 @@ public:
           send a series of commands to modify various settings to stdout. The
           currently recognized commands are:
 
-            - `extra-sandbox-paths`  
+            - `extra-sandbox-paths`\
               Pass a list of files and directories to be included in the
               sandbox for this build. One entry per line, terminated by an
               empty line. Entries have the same format as `sandbox-paths`.