doc: consistently refer to 'fixed-output' with a dash

General cleanup that makes it easier to search for the term.

doc/manual/command-ref/nix-store.xml

@@ -936,7 +936,7 @@ $ nix-store --add ./foo.c
 
 <para>The operation <option>--add-fixed</option> adds the specified paths to
 the Nix store.  Unlike <option>--add</option> paths are registered using the
-specified hashing algorithm, resulting in the same output path as a fixed output
+specified hashing algorithm, resulting in the same output path as a fixed-output
 derivation.  This can be used for sources that are not available from a public
 url or broke since the download expression was written.
 </para>

src/libstore/build.cc

@@ -3155,7 +3155,7 @@ void DerivationGoal::runChild()
                 // Only use nss functions to resolve hosts and
                 // services. Don’t use it for anything else that may
                 // be configured for this system. This limits the
-                // potential impurities introduced in fixed outputs.
+                // potential impurities introduced in fixed-outputs.
                 writeFile(chrootRootDir + "/etc/nsswitch.conf", "hosts: files dns\nservices: files\n");
 
                 ss.push_back("/etc/services");